This is probably a situation where the good old-fashioned Yellow Pages beats the internet.
Try looking under 'Engravers' and you'll probably find several firms in your area which offer this service.
Additionally, take a look under 'Trophies, Medals & Rosettes'. Most of the shops which sell sports trophies offer a glass-engraving service.
Also, around Christmas time, department stores (e.g. Debenhams or John Lewis) sometimes have in-store engraving promotions.
Some shoe repair 'heel bars' also engrave glass.
Make sure you get a quote before agreeing to have the engraving done. I've not had any engraving done on glass recently but, a few years ago, I bought stainless steel ballpoint pens as presents for two friends. I then took them to a heel bar to have their names engraved on them. The pens cost �6 each. The engraving (9 letters on each pen) cost �12 per pen!
Chris