I replaced all the radiators with bigger doubles and installed some new ones in rooms that didn't have them.
According to the data you provided later, you now have 9.6kW of rads (BTU are no longer used).
Do you know what your actual requirements are? If not, use the
Whole House Boiler Size Calculator, but set the Domestic Hot Water Allowance to 0. This will give you the central heating requirement, which I would like you to post.
While the CH warms the house we haven't managed to get it to the desired temperature yet. I had it set to 21 degrees for 6 hours today and it only reached 14. The flow temperature on the boiler never exceeded 53 even though it's set to up to 75 on the boiler. I have the weather sensor 470f which I know overrides the flow temp on the boiler but I would have expected the boiler to put more effort into warming the property. If the desired temp is 21 and temp on the thermostat says 14 then surely it should ramp up the flow temp to try increase it, that doesn't appear to happen. I'm concerned that the flow is causing the problem.
I suspect that the reason for the house not heating up properly is that the boiler is cycling due to it being oversized compared to the total rad output.
You have 9.6kW of rads fed from a boiler which can produce between 5.5 and 25kW. It is starting off at max output, but is unable to modulate down fast enough, so the water temperature rises very quickly and the boiler has to cut out and cool down. The average temperature is therefore much less than expected, so the rad output is less.
The solution is to restrict the max output of the boiler to say 12kW
As for pipe sizes, it all depends on what temperature differential you use. The boiler expects a 20C differential, so you should really be balancing the rads at 20C. The result from the boiler calculator (link above), when compared to the total rad output, will tell you if this is possible. The rad total need to be at least 20% greater than the heating requirement to allow a 20C differential.
If this is possible, there is no real need to change any of your pipes.