I'm not sure that's true about the UK and prescription glasses. When I moved back here I packed my glasses up in storage so was going to be without for 6/8 weeks before our stuff arrived. I went onto Glasses Direct[1] and ordered 2 new pairs of glasses for £50 by putting in my prescription details from another country. The glasses themselves are regulated as medical equipment, but you could go on there and buy any prescription you want and nothing will stop you.

[1]: https://www.glassesdirect.co.uk/