Another vote for Hydrotacs. Excellent product.some on amazon ..
Rapid Eyewear BIFOCAL READY POLARIZED SUNGLASSES + 2.5. Corrective Sports Glasses for Running, Cycling, Golf, Fishing, Driving and DIY etc. For Men and Women. Built in Reading Segment: Amazon.co.uk: Health & Personal Care
or if you already have cycling glasses then how about these
Stick-On Bifocal Magnifying Lenses Hydrotac - Reading Lenses for Sunglasses (CLEAR, 2.00): Amazon.co.uk: Health & Personal Care
Been using this type of stick-on bifocal for ~10 years. Excellent. Allows reading of maps, gps, etc. via ordinary cycling glasses.Hydrotacs are brilliant if you just need readers. They stick very securely and you can swap between lenses/glasses.
I second that, Colin. Boots opticians quoted me £350 when I enquired about bifocal cycling glasses. I ditched the idea fast. Then came across the BBB ones for about the same as you paid and they do exactly what I wanted them to do. The magnifying panels are discreet and unobtrusive but allow me to read maps when I want to. It did take me a few rides to get used to looking down to see what gear I was in on the cassette and whether my rear tyre looked a bit flat. It can be difficult not looking through the magnifying part. You soon get used to it though. Very good value for the money.