I can recommend the TrustFire G02 CREE XP-G R5. You can get them on eBay for about £11-16. You need one CR123A battery, a rechargeable one will cost another £7-9.
It is a basic small cheap tactical flashlight that fits onto a 20mm RIS picatinny rail. I have mine mounted under my barrel and in easy reach of my foregrip to pressure switch on the 600 lumen light/strobe. It is small enough to fit a pistol. Being 600Lm it is bright. Only on, off, and strobe.
Only negative is the lens is not BB proof, had mine shot out at The Mall, but the flashlight still worked. Will get a thicker filter to protect it.