Purple toadflax. I have it in my garden. It seeds freely and is a bit tricky to pull up so if you don't want it popping up all over the place I'd get rid of it before it seeds.

Agree, linaria purpurea. Toadflax. It can work for some tall wispy color if you are willing to deadhead, prevent self sowing.

It's only a weed if it's growing where YOU do not want it! LOL

It isn't native in the British Isles but has become naturalised from the European mainland. A very common self sower in gardens. Your decision as to whether to keep it.