I have highlighted some great deals from Amazon UK here. There are selected guitar strings for classical, acoustic, electric and bass guitars. All are chosen from personal experience and most have 5 star online reviews. There are also some bulk string buys.
When buying guitar strings remember they come in different tensions or guages. Thin guage or soft tension strings are kinder on your fingers, easier to bend too, but the lack of string mass reduces volume and tone. Higher guage strings are much harder on fingers and sometimes almost impossible to add vibrato to or bend but they sound louder and fuller. This is why many great players use heavier guage strings and then detune. Stevie Ray Vaughn's amazing tone was due to the punishing string tension he used. If you are playing grunge or downtuned metal styles you also need heavier strings.
Nickel strings are suitable for electric guitars but can be used on acoustic guitars at the expense of tone. Phosphor bronze strings are unsuitable for electric guitars due to poor conduction but sound great on acoustic guitars. Nanoweb coated strings are amazing. I use them all the time because they increase string life so much they actually work out cheaper. What is more, they don't lose tone.
Please remember steel strings will quickly destroy a classical guitar - just DON'T DO IT! If you want an acoustic sound, buy a cheap acoustic guitar!