admob is quite right.
The problem lies with the networks and the operators.
BBC did a coverage of UK mobile coverage though "crowd souring" in 2011. During the summer they followed 44,500 users and logged their mobile access though a mobile application where they noted that the average user only had access to 3G network 75% of the time (you can find the details by googling "BBC" and "Mobile Coverage"). If you instead google "OFCOM" and "Coverage map" you'll find on their website a pdf file with maps that shows the technical coverage for all UK mobile operators. hopefully that might help you determine who you think is the best UK 3G operator.
So, why is mobile data coverage so poor in many place in the UK? Well, let me explain.
1. Most UK operators were eager to comply with regulations for the 3G licences but many put in place a bare minimum of coverage for the "new" 3G networks. Following rollout, the telecom "bubble" bust and network expansions and upgrades beyond meeting regulatory targets were put on hold. The assumption was, and is still for many operators, that 2.5G works just as fine (GPRS and Edge) as 3G and this has also been indirectly acknowledged by Ofcom and hasn't helped the case of the users. The coverage requirements laid out for the operators said that they were obliged to provide cover "to an area where at least 80% of the population of the UK live" and and also meet minimum speeds for service: "be able to provide a range of applications such as voice, text, video and multimedia services for outdoor reception with speeds up to 384kbps." Operators claim today that they cover 90% of the population....according to their own statistics. Ofcom conceded in 2007 that the requirement for speed may only be applicable to Urban/Suburban areas and is linked with the high costs of rolling out 3G base stations for low density populations (higher frequency than GSM = more transmitters to achieve same coverage). However, no operator have yet to be sanctioned by Ofcom (where penalties involve possibly losing the licence) for failing to comply with coverage or speed so most operators today are pushing the enevelope to see what they can get away with. Thus, network coverage is not as great as it should be.
2. Following the first point, since the sale of the 3G licences much have happened on the market. When these licences were sold mobile data was limited to GPRS (which was still a "new thing" in those days) and mobile internet was mostly still a dream. Most phones that could interact with internet used WAP interfaces and ultra slow non-graphic WAP pages. Then the SmartPhone happened. After the 3G licences were issued, these devices were heavily marketed and now everyone has one. These phones use data in so many ways but like a highway, there are only so many users you can fit in the limited space provided by both transmitters, backbones and servers. The weak link is the BTS or the transmitter. When the number of users on one transmitter begins to reach it's limit, the transmitter begins to reduce the available speed (or bandwith) with all users which can in some highly polulated areas give only crawl speeds for mobile data. Another thing with 3G is that congestion on a transmitter ALSO means that the coverage area decreases (splitting transmission power to many devices) and some users then end up at the outer reaches of the transmitter where speed is slower - the closer you are to the limit of the transmitter coverage, the slower your data speed will be. Imagine an umbrella when it's raining. You can only fit so many people under the umbrella. So when when it starts raining heavily, those near the edge of the umbrelle will get wetter (slower data speed) and those near the centre will be drier (faster data speed). But the 3G data cell (coverage area) shrinks when under high load to be able to provide better service to the users it CAN service thus the umbrella (using the same analogy) shrinks ! There are several examples of this situation inside central London where you would imagine great coverage. Given the economic downturn, mobile operators have been forced to focus on ARPU (average revenue per user) increasing activities and selling more phones to more users at lower proces turns out to be a rather dubious business model (as if we didn't already know that). The more smartphones they sell, the more data users there will be. More data users but less revenue (most users want mobile data for free) and no money for upgrading or expanding network structures...... it's a killer combination for network congestion.
3. Following the first two points, UK operators apply technical settings such as Grade of Service (GoS) and Quality of Service (QoS) which are some of the lower one you'll find in Western Europe. On average across a network blocking on mobile networks should not exceed 2%, meaning that the probablity of a call being blocked or rejected should not exceed 2%. Reality on cell-level is different where during peak hours some cells can reach and exceed saturation levels. Although engineers cannot control how many people WANT to use the phone at any given time or place, there are statistical models that they can use to forecast EXPECTED peak loads which would in turn determine the design and quantity of mobile cells required to provide adequate covererage for any given area. Remember that mobile phone companies like all other businesses operate for profits and will normally only roll out network coverage which meets regulations (that normally only set minimums) and user expectations (users normally vote with their feet when expectations aren't met). SO, Network standards will only be as good as their customers allow them to get away with hence the great disparity between European countries.
Although this doesn't fully answer your question with regards to which operator have the best 3G network in the UK, it does give you an idea of the problems facing the mobile industry and users in the UK. At the end, you make up your mind and vote with your feet.
