I'm not sure there is a market for managed VPS's?
I've used Rackspace vps's extensively (unmanaged), mostly the 512Mb / 1024Mb packages as are excellent value at just under £15 per month for the 512Mb package and performance for £15pcm has been excellent. Looking Rackspace do a managed vps to which is new to me
http://www.rackspace.co.uk/managed-cloud/ , you pay for it though.
http://www.bytemark.co.uk/ are supposed to be good to. Not used them personally but know someone who works for them and hear good things.