Looking at the rack servers is great, but do you have a rack? Have you got space for a rack? If not that's £1600 on top for a 24u rack and then finding a space for it.
Where do you plan on siting the server, trust me, you wont want it in the same room as you as the whine of the fans will soon get on your goat, even if you do go for the tower it will be loud! Steer clear of blades and blade enclosures for this reason.
Do you have air-con sorted out? Running a server in the middle of summer when it's 25-30c ambient air temp in your office will get the server nice and toasty! Without good air-con, again steer clear of blades. (again, i know the jokes about UK summers, but if you are serious about being in business you need to consider this)
What about power supply and UPS, servers are a fair draw when running? And if you have a power cut you need something to keep things running whilst they shut down nicely, rather than a lot of blue screens and data loss.
What about switch(es) to plug it all together?
Looking at the Dell site, i'd personally say go down the route of the T710 range if you still want to, it's a tower, can be tucked away and also converted to rack mount in future if you do get rack space later on. Dual processors, 16gb ram, 6 HDDs, dual 4 port ethernet, dual power supplies comes in around £4k + VAT. (and that's with 3yr next day support, do you need same day? if so, add another £800)
You gave a small smile when asked about O/S, it's not to be shirked away from, it's not only the O/S you need to think about, but also the VM software (VMWare isnt cheap!)
Also for internet, there are a few options to look at, but to give you an idea on pricing for a semi-decent setup, i just got a price for a 10mb internet connection hosted at a data centre that is delivered to my sites by two 10mb bearers giving 5mb across each (if one fails the other speeds up to 10mb to ensure i get the bandwidth i pay for). Set up is around £10k and annual rental around £20k.