As others have said it’s probably an infected bite you have scratched.
I’m not sure if any bugs in the UK have nectrotic venom but it looks similar to necrosis. Your options could be a bite from a false widow Steatoda nobilis which are native to some parts of the UK but dont really turn black. A more exotic option could be a Mediterranean recluse spider bite loxosceles rufescens . They are definitely necrotic but probably not native to the UK. If it was a false widow you would probably have felt the bite. If it was a recluse you might not notice right away. The recluse bite is significantly worse out of you two.
If the hard area around the hole starts to turn black then it could be your second option. Which is good as it might show the UK is getting warmer. 🙂