Split tax year treatment?

Talk to the official tax authorities in both countries.

Back in (sometime in July?) when you arrived in Belgium, did you commence steps to become resident here, i.e. did you go to the commune and ask for an ID card)? "Normally", if you plan to stay in Belgium for more than 90 days, you need to inform the communune within the first 90 days.

Your residence status and your tax status are not necessarily linked, but it would certainly help with establishing dates of when you arrived here and when you left. If you weren't actually "resident" here, you could easily claim to have been resident in the U.K. throughout, and just pay UK taxes.

I would speak with the tax authorities in each country as J suggests above. Starting with the U.K. authorities, as that is where you should declare your income first (as there are lower income tax rates).

However, it could well be worth your while speaking to an accountant, as you may be able to take advantage of two personal tax allowances, one in the U.K. for your earnings in the UK, 2020 / 2021 tax year and and one in belgium for the 2020 tax year. Depending on how much you earn, that could easily be worth a few thousand, so spending a couple of hundred with an accountant is probably worth it.

If you were Belgian tax resident in previous years you must have registered, if you failed to deregister by returning your Belgian residential card to your (former) commune, you will inevitably still be considered as a resident and therefore subject to Belgian tax.
If you actually talk to the tax authorities in both countries they will usually come to an equitable agreement. You might find it helpful to actually read the double-taxation treaty which is available on-line.

> I read in the expat tax advices
How did you become fiscally resident outside Belgium? In what country?

It sounds like you've tried to do this yourself and have screwed up.

> Belgium now want to tax and fine me +50% for part of the year not earned in Belgium..
I should bloody well hope so too. You are liable for tax on all your income worldwide as a Belgian resident. Being non-resident is a complex tax set up and you need professional advice to do it properly. It only works for a small minority.

It's not at all clear from your initial question, and your subsequent answer, what your situation is, so it's difficult to give you more advice here.

However, if you were resident in Belgium, and then left after 151 days of the tax year, so around the end of May(?), then you will only be liable in Belgium for income tax on the income you earned in Belgium while you were resident there.

From early June, you would then be liable for income tax from your new country of residence (presumably the UK?). This is really simple 1st year tax knowledge that anyone (including everyone at the Fisc) knows.

So I'm inclined to think that your situation is somewhat more complicated.

@SCOTTIE the rule of thumb is that if you work for more than 183 days in Belgium then you are liable for taxes here. You have not specified whether you spent the 150 days working remotely a UK employer or got a new job in Belgium, if it's the former, you are not liable for taxes in Belgium.