I am in the UK right now and wondering if the affirmation which is required can be obtained in the UK? Thanks in advance

No, you can only get it from your embassy or consulate in Thailand.

(The signature from the consular officer needs to be checked by the Thai foreign office, which only has the signatures of consular officers from embassies/consulates in Thailand).

When did it change? I got a document from my local registrar and the Amphur was happy with a translated version 10 years ago tomorrow.

Nothing changed, your amphur was probably not aware of the rules.

Indeed.

We married nearly 13 years ago at Bang Rak and they told us I can only have the AFM witnessed at my embassy, not in the UK.

But they are used to dealing with foreign marriages.