This may be of interest. http://www.englandrugby.com/mm/Document ... eutral.pdf
I'd definitely talk to a registered dispensing optician, in some high street opticians the person selling the spectacles does not have to be registered or qualified, however all sales of all glasses to U16s should be supervised by a qualified and registered person. Looking at the RFU position They have to be supplied by a registered optician. I'd definitely stay away from online suppliers.
Contact lenses are often a better option, but also talk to your optometrist, children who wear glasses in class often manage sports just fine without. Don't assume wearing goggles is safer than not.
£80 may seem a lot, but compared to boots/custom gum shield/other kit, for something specific for you and to protect your eyes...