£20/£30 is probably a budget for one light: front or rear. Spend more on the front, i would say. The rear doesn’t need to light the road up, it just needs to warn drivers behind you.

However, the front may need to light up the road ahead, although the extent of this depends on the types of roads you’ll be riding on. If they have street lighting then you probably don’t want the brightest light possible, but more of a ‘be seen’ type of light. If you ride on unlit cycle paths and roads then you’ll need to light up the road ahead for a reasonable distance. Also, don’t forget the light spread to light up the sides of the road.

Battery type is also important, as is the number of hours of light you’ll get out of a full charge. A lot of lights have a lot of lighting modes, but some of them you’re never really likely to use.

Wiggle and a couple of other UK sites have a light comparison tool that will allow you to see the range and width of the light pattern on a side-by-side comparison.

I’m also shopping for a set of lights so it’ll be interesting to see what you decide to buy.