Simply: what is the best network in London for data-based phone use (and will it all change once all four have LTE)?

So I'm on a (freely bought) Galaxy Nexus, so this is a two-part question I guess then. I want to fix things now, and I want to freely change to a new LTE phone and (if necessary) new, best LTE operator 6-9 months down the track too.

Currently, I'm with T-mobile(/Orange/EE) prepaid, and uh, I'm now realising there's better.

My criteria/needs:

- Cheapness. I prefer prepaid if possible, or at least no silly binding 6, 12 or 24 month contract. I HARDLY use my phone to ring people, I just accept calls, use data with skype and viber when I have to make calls, conservatively text and even try to use whatsapp and email as much as possible now. I just want my SIM account to be a pipe nowadays, really. Makes total sense.

- Unequivically, unlimited, data. So I do tethering a lot with my nexus, and for a while I had this wonderful £5 for unlimited internet thing that I could activate by 'texting MONTH2WEB to 441', it seemed too good to be true, and yes it was, cos I later got this 'you have reached your fair usage lmiit' text about 15-20 days into it...and now, it seems the offer has finished anyway, it doesn't work anymore when I text it, so pfft, t-mobile.

- Best coverage, speed and reliability. So I dunno why I haven't thought about improving this, but it came to my attention the other day, which is why I'm posting this thread. I was in level -2 in a large building in king's cross area, in an enclosed central room where I of course, didn't expect to get any resolution. Maybe I'm used to this and things could be better. BEcause the colleague next to me, on an o2 phone, had FULL bars of reception and was chatting away on it!!!

I've since then tried to google to figure out what network has the best coverage/penetration (in london), and also who offers truly unlimited data (without some £20+/month plan type setup) while I'm at it. but I've been in endless circles of whether it's Three, O2, or Vodafone. (or even my ol' T-mobile, after all.) See I DO travel sometimes elsewhere in UK, as far up as Scotland, but that's far and few between so I'd rather go with the network that has incredible, tenacious glue-like reception in London wherever I am. And unlimited while the cheapest too. (Sounds like Three, but then I read comments like this.)

So which is it? Which is it now, and are there any pointers about what to look for once it's all an-LTE world later?

Productivity is important, I really wanna just go with the best that exists in London, right now, it's time to optimise. I guess I can just buy a prepaid sim for whatever you recommend (maybe one for Three, O2 AND Vodafone? they're only £2 each, then a bit of top-up)...Also I have friend, (not a techie) who keeps on pestering me about giffgaff but I'm inexperienced about how reliable and long-term VNOs really are and the whole thing is going to change in 6 months' time anyway...

Thanks