Yes all travel is Visa dependent. Thus far the UK government has only lifted the 14 day self isolation requirement with countries that offer a reciprocal arrangement. With Thailand extending it visa amnesty again, I think that indicates there is no plan to open the borders so no reciprocal arrangements will be coming with Thailand soon. Thai citizens can leave but as has been said by others, your girlfriend will need to go through the quarantine on her return to Thailand.
100% quarantine (as it stands today). Only “Air Bridge” countries can avoid it. Thailand is not on the list of Air Bridges. If you have been to any other country in 14 days prior, even if it’s safe, before entering the UK, you will be required to name an address of where you will self isolate for 14 days.
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