At £150 anything with full suspension will be a BSO, and isn't worth it. For your budget, you are looking at the very bottom of the lowest spec bikes out there, anything from a Supermarket, mail order catalogue, Argos, should be avoided, the best bikes for £150 are going to be from brands like Decathlon,
http://www.decathlon.co.uk/rockrider...d_8238214.html if you can stretch a little to the £200 mark, more opens up, like this
http://www.halfords.com/webapp/wcs/s...egoryId_165534 This isn't an MTB, but from the sound of your post, is what you need rather than a MTB.
If you want an MTB to go off road, you will need to increase you budget at least 2-3x and be prepared for part needing to be replaced, off road riding isn't cheap, also you haven't noted anything like helmets, gloves, eye protection, shoes, clothes, if you want to go off road, you need to budget for all of these as well.
For the ideal of 2nd hand, we don't have the same market as the US in used bikes, if looking on ebay, anything good will go for reasonable money, making, Gumtree is an option, but there is a lot of junk on there.