Specifically, not wanting to spend over 600 sterling pounds (including
VAT) on a condensing boiler (ideally SEDBUK rated 'A'), what
makes/models does anyone suggest, and more importantly from which
supplier?

You are getting close to the price points of some good makes. I would
suggest pushing just a little further (100 to 150 extra) to get them rather
than no-name stuff. By the time you are paying 700-750 quid inclusive, you
get the choice of the Ideal Icos, Bosch-Worcester Greenstar and Glowworm
SXi, for example. They all include most (or all) of the components for
sealed operation and even a circulating pump. Some have cheaper versions
without the pump or sealed components.

Ensure that what you buy has a down firing burner (or other burner design
that doesn't soak the burner in condensate). Ensure it has only a single
main heat exchanger. (Combis will also have a secondary water to water heat
exchanger).

Also, as I am running around trying to get prices for plumbers to
change my open vented system to a sealed system, what should I be
looking at paying for this conversion (london area).

This is part of the boiler installation. Most condensing boilers (but not
all) are system or combi types. Both will have all (or most) of the
components for converting to sealed operation. The only thing that must be
done is to cap off the old vent and feed arrangements and remove the pump.
You then connect up the pressure relief and mains water supply to the
boiler. Sometimes, the mains connection is not included, but this costs a
tenner from the plumbers' merchants.

Christian.