Don't get a 125 or moped, you will be very disappointed. But then again the 125 will save you loads in insurance compared to the golf and is no where as faggy as a teenager on a moped. Last year at 21 I got a quote under £600 for a Yamaha YZF-125, which compared to a mk4 golf of £1600 a year was cheap (first year it was £2900 so I really feel for you).
Also look for bargain bucket cars and just keep getting quotes as the insurance industry really makes no sense. Like if I leave Admiral now and rejoin my premium jumps to £1900 a year, if I stay its £900 a year, as if its justifiable in any way.
But try the derv route as it sounds the best possibility. I would say a 1.4tdi 6n2 polo, but they are out off the budget reach, plus if you look out for the correct reg to a later one you will save on road tax, something else to bear in mind.
