What recommendations have you already been given so far. We work out of service offices so don't really the same needs as yourself.------------------------(Accounting Help)
You can try these guys: http://www.connect.co.uk/
I worked for guys that used them before. They're fairly good and reliable, plus they're located in Canary Wharf with engineers all over the place. So they have London pretty well covered.
I am working as a freelancer in London. 10 years of experience in IT administration (MS Certified) plus virtualization/cloud technology and Microsoft Office 365 which is a very popular product among small companies in terms of price and as a technology.
Let me know if you are interested.