Reform UK mainly exists to force the Tories to move further to the right, just as UKIP did pre-2016. FPTP has helped in this context, as UKIP/Reform effectively hijacked the Tories to achieve this; under PR this wouldn't have been necessary. The left wing equivalent was Momentum hijacking Labour.
PR would actually help moderate politics, for all the concern it will further enable more extreme parties like Reform. It will make hijacking other parties less effective, if at all, while also parties will have to broaden their appeal, which usually means moderating their positions, so extreme positions are less prominent. Several right wing parties in Europe, particularly in France and Italy, have had to moderate key policy positions to appeal to more voters, this is particularly noticeable with regard to their EU stance, where they no longer advocate leaving or even having a vote on the issue, despite previously advocating them.