What sort of budget did you have in mind?
Life Jackets range in price from around £40 for a manual inflation, to around £200 for automatic offshore sailing vests with harnesses.
If you tend get in and out of the boat quite a bit, accidentally triggering an automatic is a pain and expensive to re-arm, so a manual might be a good option.
For everyday inshore use, I have an Ocean Safety Kru Sports Pro manual. It is 150N approved but offers 180N of buoyancy so good if you are a bit heavier (like me!). It is also very comfortable as it incorporates a zip front and doesn't flap around too much. Mailspeed Marine is currently offering this for £69.99 against a RRP of £95.99, so a good mid-range choice.
Hope that helps.