I just flew Qatar Business Class Venice - Doha - Bangkok and they insisted on seeing a valid flight booking leaving Thailand and the flight number too!
Qatar and Air Asia are the worst for this. Emirates never asked in January and Thai Immigration have never asked even once!
UK Specsavers is alot cheaper. I've just paid 2500baht for 2 pairs of glasses with them which are varifocals, thin and light, one sunglasses with extra blue filter, one reactive with extra digital area.
Basically every option you can get and it was still so cheap there!
Unless you are buying a property or making a big purchase £10,000 or more, the exchange rate is an irrelevance because Thailand is cheap anyway.
I always convert money when the exchange rates are good. I converted a few thousand at 46.58. I'm quids in, in theory.
But on the amount I spend in Thailand it's not much.
Oh and for you lot complaining about Brexit, sterling is heading for $1.45, €1.30 within a year and is already super strong against currencies as diverse as the Aussie$, Mexican Peso and Norwegian Krone.
Nothing to do with Brexit. Simply to do with base rates in each country and their future prospects.