Redbus, Telehouse and Global Switch are 3 fairly popular datacentres in London. The actual datacentre you are in doesn't often make an awful lot of difference unless you have very specific requirements, your choice of host is usually a lot more important. It all depends on what you require I guess. You'll find plenty of companies on this board who will be able to provide whatever you need.
With regard to hostrack.co.uk, a quick trace shows that their site is hosted on the ev1 network in the USA, as opposed to London so if you are after a fully UK based host I'm not sure if you have found one yet.
I'd agree with rob on the above, would add in to make sure providers are properly multihomed with their own routers and AS number. Check to see if they have this or if they are just telling you they do because thats what their upstream has.
It depends mostly on how much space you want and what level of support.