2) Costs for conversion of open-vent to closed system (London)?

Is this DIY or paying?

Provided that the system is up to the increased pressure, it is a very
simple job. You remove the old tanks and cap off the old feed/vent. You add
the pressure vessel, filling loop, pressure relief valve/pipework and
pressure gauge. That's it. Normally, the majority (or all) of these
components come ready fitted and plumbed in the new boiler. You just stick
the pressure relief pipe out of the wall and connect it up, ensuring it
terminates at a safe location.

BTW, the 835 is a combi boiler. Is this what you want? Your old system is
probably a storage one. If you like baths, you might want to keep the
storage system. However, the combi side will be better for showers. OTOH,
some high output combis are unable to modulate down particularly low, which
counts against them as they will cycle when doing the central heating.

Christian.