The Budapest Memorandum was signed by Russia, Ukraine, the US, and UK. It remained in efect in 2008 and beyond.
The Budapest Memorandum was a Memorandum, not a Treaty. It contained non-binding assurances, not the binding guarantees found in treaties. Diplomatic linguistics matter.
Ukraine was assured of military assistance only in the case of a nuclear attack upon Ukraine. In the case of all other attacks, Ukraine was assured that the matter would be taken up before the UN Security Council. The matter was taken up before the UN Security Council which did not authorize any military action.
The UK and US are not going to war over the Russian invasion. We are supporting Ukraine. Our honor matters diplomatically and in any future attempt at denuclearization.