This is a good question. And, no, sunglasses haven't always offered UV protection. And actually some modern sunglasses don't necessarily meet good UV guidelines, depending where they are made. If your pair are branded, you may be able to research/refer to that company about their standards at the time. For instance I'd be happy wearing vintage Polaroids.
I've also read some information that suggests that if the UV protection is a coating, it may have worn off over time, but I don't know if this is really true.
You are in the UK (like me), so here's some UK information, from a vintage sunglasses site klasik.org:
A law was brought about in Britain in 1987 that said that all new sunglasses had to comply with standards of uv (ultra-violet) protection. Therefore, every pair made after that date that we have on this site will give full uv protection.
All pairs made before 1987 did not have to comply with any laws and therefore a few do not offer uv protection to this British Standard. These sunglasses are to be worn at the risk of the wearer and are sold as vintage pieces of historical interest and not as sunglasses offering uv protection.
Most pairs made before 1987 do meet today's British Standard and the branded pairs, eg, Ray-Ban, Polaroid, American Optical, Zeiss etc. are comparable with the best lenses that you can buy today.
Klasik also offer a testing service, though this may only be for their own glasses, but you could ask.
You could always have your sunglasses reglazed, this is relatively cheap, but at your own risk as far as the frames go. I've had lots of vintage frames reglazed both plastic and metal, and had no problems. However they were frames designed for optical lenses, and so made with reglazing in mind, and maybe more resilient than sunglass frames that were not intended to ever have the lenses replaced.
Search 'reglaze sunglasses' to find online companies who'll do it cheaply by mail order - I always use Ciliary Blue for prescription reglazing and can recommend them. Most companies offer this service for prescription lenses, but I'm sure some can offer plain sunglass lenses as well - I know Ciliary Blue do.
It's definitely worth taking care of your eyes!
