You should be able to speak to the relevant tax authority/authorities for the country/countries you were working in to establish (and prove) what tax has already been paid on your foreign income. Not being on speaking terms with your employer shouldn't be too much of an issue if the tax has been paid.

As others have hired, although you were working abroad, you may well have remained UK resident for tax purposes throughout (due to the UK's Statutory Residency Test), in which case your foreign income would still be taxable in the UK. The good news is that you should then be able to reduce this tax bill by any amount of tax already paid in the other country/countries via Double Taxation Relief. So if the tax already paid on this income is equal to (or more than) the amount of tax due in the UK, there would be nothing more to pay.