I know running a spur from a double socket is ok. I want to run a fused spur for underfloorheating.

Question is can I just break into the flex before the socket with a wago junction box and run it to the fused spur or does it have to be spurred at the outlet?

Reason being is like to avoid having to chase out the wall to the socket and probably have to remove the skirting.

Yes, with a few caveats.
Presumably under the floorboards? Then you’d really want to use maintenance free connectors and box. Wagos will do this.
Under underfloor heating? Not great if you ever need access to it in the future. They should be ‘maintenance free’, and last for ever, but it is always nice to have access, just in case, say, you have a water leak, and water gets into the box.

Yes under floorboards planning on using wago boxes. The UFH is in the next room (en-suite) so fused spur is on bedroom wall.

The rest of the house seems to be wired with standard round junction boxes under the boards, as was my last place. As I’m refurbing a room I’m swapping for wago boxes.

If your using maintenance free connectors like wago you still need them to be accessable for inspection. Running a spur is ok unless the socket you want to use is already a spur, if it’s a radial circuit use 4mm not 2.5mm.