Very few companies in the UK will offer a fully managed server for £100 per month. The rackspace/power alone in a good docklands facility where a lot of well known 'managed' providers are based would cost almost 50% of that.
What level of management do you require?
What OS do you need?
What server specs do you need?
one thing, don't go by price first, its worth also trying forum2.co.uk, one alternative could be that you get un-managed solution and then get outsourced support, it may be cheaper, but best to look around first