hi Nigel,
If you can avoid registering.. then I would do this… Mainly as registering and de-registering requires filing in a lot of forms (= queing for hours in various offices if you are not hiring a gestor/accountant to do this for you). Probably more hassle than its worth (I was autonomo for 5yrs and it cost via my accountant everytime I registered/de registered - partly as I am not 100% fluent - but you can do it yourself if you want all the aggro!). Probably best as Robi says.. declare the income in the UK.. (I have been told the tax man here can only chase you for 4 yrs before it is expunged. They have to find you first 😊!
sj