New Boiler & Smart System Recommendation

spicymchaggis

First off appreciate anyone who takes the time to give some feedback here.

So I'm looking at replacing an ancient Grant 50-80 slimline oil boiler that is an external boiler house in a house that was built in the early '80s. It takes a good amount to time to heat the rads in the house, a flush would probably help.

The current system in a single zone, all on or all off. Getting multi zones fitted would probably involve extensive work.
We recently had a gas line run to the house.
There are 8 rads in the house, 3 downstairs, 4 upstairs, and 1 in the converted attic.
I'm looking at switching to a combi boiler as we only have one bathroom and the sink in the kitchen.
Are there any recommendations as to which boilers or a different type?

What type of smart system would recommend for a setup like this? Hive / nest / evohome / Drayton etc...
Recommend smart TRVs?
What grants are available for a configuration like this?

Once again, any help appreciated.

spicymchaggis

If dual zone cannot be fitted due to extensive work would fitting all rads with smart TRVs count for control upgrade grant?