If I can get my sealed system up to at least 1.5bar by filling it from the
mains filling loop, does this mean I have enough pressure to fit a combi
boiler?

Yes it does. 1.5 bar is easily enough for a combi boiler. However, it
doesn't tell you if the flow rate is good enough. Flow rate is more
important than the pressure. You mention 9 lpm elsewhere, which is not
enough flow rate. However, this poor flow rate might be for a fixable
reason, such as a broken/partially closed stopcock, or a modern styled tap
with very narrow waterways that is not designed to pass more than that flow
rate. (I'm not suggesting that the tap would need replacing, just that it
might affect the feasibility test).

If you have a garden tap, try measuring the flow rate off that instead.

Christian.