I'd go for a inexpensive, but not too cheap, modern condensing system
boiler
such as an Icos or Glowworm, plumbed in S-Plan.

P.S. Approximate cost (inc. VAT) would be

Boiler = 700
Pump = internal
Bypass = internal
2 x 2 port valves = 70
Programmer = 35
Programmable Thermostat = 30
Filling loop = 10
Radiators 6x60 = 360
TRVs 10 x 10 = 100

This leaves approximately 200 quid over for pipe, fittings, cable and other
sundries, but assumes your existing tank and radiators are adequete. Note
that the serial radiators will require fixing if you want the TRVs to be
added to them, although you might get some results by adding one to the
"cold" end of the chain.

Christian.