How do I buy Tazorac (tazarotene) cream when they don't sell it or the active ingredient, here in the UK anymore?

Try online pharmacy. You may be able to buy your preferred formulation from an online pharmacy with a valid prescription. As long as your prescription is valid, there's nothing wrong with getting your Tretinoin from an international pharmacy that still formulates it, assuming it can get through customs in your country.
On line. You can get medications from a Canadian pharmacy. They will ask you to mail or fax a copy of your prescription from your doctor.