Do not go with British Gas under any circumstances. You WILL get ripped off!
As part of my families Heating business i can give you an idea
A 33KW combination boiler is around £800-£900 purchase inc VAT
Depending on where it will then be located you may need flue extension kits, perhaps roof flashing could possibly be between £100-£200 if required
Go for around £100-£200 for various pipe fittings
New time-clock and thermostat up to £100 depending on the type, wireless? etc.
Then labourwise is entirely time dependent
For a straight swap with little moving of pipes it can be done in a day
Replacing an open vented system would mean removing hot water tanks and moving pipes so you could be looking at 2-3 days
Don't expect to pay much less than £250 for a 8-10 hour day and if you do then you should be worried about the type of plummer you are employing. We charge around £330 a day inc VAT
So in all, i would look anything between around £1800 and £2500 including VAT for a fairly standard installation but this could vary a lot depending on your specific job and what it entails
Hope this helps
EDITED some figures