An interesting and well thought out post, if I may say so, goji. I agree that the GBP/ Thai Baht exchange rate was falling long before Brexit. I recall the exchange rate being around the 70 -74 Baht to the GBP mark when I first started holidaying in Thailand in 2003.
And, as regards the UK travel allowance, I’m pretty sure that it remained at £50 from 1966 to 1979. There follows an extract from Hansard from 1969:
From a brief Internet search I could not find anything definitive, but the following extract appears to indicate that the £50 allowance remained unchanged until it was abolished in 1979.£50 TRAVEL ALLOWANCE
HC Deb 27 November 1969 vol 792 cc657-701 657
§ 5.13 p.m.
§ Mr. Reginald Maudling (Barnet)
I beg to move, That this House regrets the refusal of Her Majesty's Government to increase the £50 travel allowance. It has taken some time and a considerable journey to reach this subject. I am afraid that our time now available for debate will be very much truncated. Therefore, I will try to compress my remarks and make them as brief and concise as I can, since I know many hon. Members on both sides wish to take part . . .
Exchange controls had been the cross that British individuals and businesses had had to bear through sterling’s long period of vulnerability. During the worst of the country’s ‘sick man of Europe’ period in the 1960s, before and after the 1967 devaluation, a £50 ‘foreign travel allowance; operated, this being the limit on the amount of money British travellers could take abroad.
http://www.economicsuk.com/blog/002127.html