Welcome back, UK! (The UK should not demand special conditions after causing cost and temporary instability by leaving the EU.)
The UK has always demanded (and received) special conditions, not just on Horizon, but on lots of other things too, even while it was still in the EU.
It all goes back to the fact that the entire EU has grown out of what was ultimately a deal between France and Germany, which has always left the UK out as the other one of Europe's big 3 countries. In fact France even blocked the UK from joining at all in the first place.