It depends on your needs and type of hosting you are looking for. By example webhostingtalk is hosted on Rackspace servers, but they provide only dedicated hosting solutions. The vbulletin host should support PHP and MySQL. (these are the requirements for vbulletin)
A hosting provider which supports PHP5 and MySQL are required.
vBulletin can at times be a heavy script once your website starts getting busy. You have taken a great step in having a great community (choosing vBulletin over the rest) however now you need to find the right hosting provider. Try to weed out those fly by night hosts. Make a list, go down the list and figure out which provider meets your needs, and go from there.
Personally I wouldn't limit yourself to the UK. Pick a US provider in the right location, and UK traffic will be plenty fast enough (you're not hosting game servers)
I would also avoid oversellers if you are planning for your board to grow at a decent speed. Oversellers can be good for a small based community, However, Be weary of their database performance later on when and if your site seems to expand.
I ran a Forum with Bluehost and their database performance was very very slow.. Unresponsive and pathetic. However, I did manage to move and that new host was quite nice and handled the forum quite well.
I do not have this forum anymore, Though just be careful who your choices are if among the top 10.
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What sort of requirements do you have? You're asking a very open question
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You should consider going with a host that doesn't oversell. Expect to pay between $20-$30 a month.
If you have a small forum. You might be able to get it on a $10 plan without any issues with performance (PHP+MySQL). It all depends on the host and if they're overselling.
If you have a pretty busy forum. Look for a VPS, and then optimize it. Depending on your skills, you could go really really far with a VPS. For example lighttpd+fastcgi+xcache+memcache, will run extremely well.
It all depends on your situation, let us know more about your site.