Best place in London to buy a SIM?

Three has the best deal on prepaid data currently. Service can be sketchy to non existent in some areas, however, as it was when we visited Peak District last year. Vodaphone is more expensive but has better coverage. Each company has their own stores and Carphone Warehouse is a good option.

We have purchased a Lyca Mobile SIM card at Heathrow twice. I believe it was in the 10 - 15 pound range. It is pay as you go. We purchased the first one in a machine on the arrival level and the second one on the lower level in a small shop on the right just before heading onto the tube.
It is convenient as then we can make phone connections when we are above ground heading into London.

hi, i bought my phone/sim from carphonewarehouse. it was close to my hotel so i didnt have to go somewhere else and take time from my sightseeing. happy trails.

I know there's definitely a big Vodafone shop in Heathrow Terminal 5. If you aren't sure which carrier to go with, Car Phone Warehouse has SIMs for all the major carriers (except they no longer seem to carry Virgin Mobile, for some reason). The salespeople there will help you sort out the carrier and plan that suits you best. Useful comparison of the current offerings here: http://www.carphonewarehouse.com/price-plans/sim-only/pay-as-you-go/regular-sims

SIMS sold in the machines at Heathrow are usually not good value because you often pay extra for the SIM itself. Better to go somewhere like CPW because not only do you only pay for the call time on the SIM but also, because there are so many options, it is really helpful to talk it through with someone.

*Data mostly* Linda makes a very good point, there are lots of options and CPW allows you to see all of them in one place. Take a look at this page for a comparison of the various SIM-only deals they have: http://www.carphonewarehouse.com/price-plans/sim-only/pay-as-you-go/regular-sims One thing that you need to bear in mind is that some, maybe all, of the inclusive data packages on PAYG SIM-only plans will be restricted to UK residents. I can only say for sure about my own, which is with Vodafone and is pay as you go, where a £10 top-up buys me that value of calls and texts plus 500 MB of data, but I checked the terms and conditions and it is for UK residents only. I'm sure that if you go into a London CPW branch you won't be the first overseas visitor they've helped with this requirement and if there is a solution then I would expect them to know it. Portsmouth - about 40 miles from where I'm typing this. The bigger carriers like O2 and Vodafone should be fine. Where I live in rural south west Hampshire those are the only ones that have any reliable coverage at all. I'd avoid 3 completely if travelling outside London. Virgin uses the Orange and T-Mobile networks and should be fine in Portsmouth, but all of the carriers have coverage maps on their websites if you want to check exactly where 3G data service can be expected.