There are about 3000 threads about headlamps on this forum, with 400 people responding with 400 different suggestions on each one.
Anyone who tells you which one to get is wrong.
You need to ask yourself the following questions:
a) what do I need it for? Diving? digging? expeditions in big caves? leading groups
b) how much am I willing to spend
specs that will be different on different lights and have different uses will be as follows:
- waterproofing levels (think diving)
- type of batteries used (think convenience of leading groups, or perhaps for expeditions)
- lighting power (what size spaces are you going to be in)
- weight (and possibly more importantly front to rear balance)
- fitting to helmet (easily removable for other uses? type of helmet to be fixed to?)
any light is going to be compromises of all these things and then finally compromising on how much you want each of these things against how much you're willing to spend.
A short list (off the top of my head) of retailers/providers that people regularly recommend here:
Starless river
Inglesport
Roy Fellows
Little Monkey
The advantage of using one of these four, is having a UK based supplier that will be able to assist if anything goes wrong with the light.
Each of these have a range of lights available, check them out. Ask questions of the supplier, ask questions on here. If you know someone that has one, ask to borrow and test it out?