My previous follow-up was rather simplistic.
If you are VAT registered you charge VAT. If your business income is below £85k you don't need to be VAT registered, though you can opt in.
Depending whether you are a sole trader or limited company, there are implications for income tax, capital gains tax, corporation tax, revenue expenditure / capital expenditure etc, and this will also depend on how you acquired the domains, whether you're holding them to resell or whether you're holding them long-term for brand protection, website income etc.
In my specific case, I have domains in my own name and limited company name. Some were acquired and treated as capital asset, whereas those I registered are all taxed on an income basis, i.e. income tax or corporation tax.
My accountant proposed an optimal structure given my specific situation. Probably best you speak to an accountant to optimise yours.