Transair will sell you a flourescent jacket for £10, Tchibo will sell you a similar one for £4.
As far as life jackets are concerned, marine jackets are auto-inflation, avaition jackets are manual inflation. The theory being that if you fall off of your boat, the jacket will just inflate on contact with water, but if you ditch your aircraft, water may come into the cockpit before you get out, causing an auto-inflate jacket to blow up. An inflated jacket may prevent you from escaping, hence avaition jackets are manual inflation.