Cheapest 2 seaters will be microlights. At around £4-£7k there's loads on the market; if you prefer 3-axis controls, reckon on something like an AX3, AX2000, X'Air or Thruster. Running costs in the order of £35/hr. You can clearly spend a lot more for a more sophisticated aeroplane (or less for something crude and slow but safe).
Cheapest 2 seat "group A" will be LAA (a.k.a PFA) Permit aeroplanes. Good stuff starts around £7k, similar running costs to microlights.
PA38, C152, etc. are old technology, expensive to run, and daft choices (in the UK) for a personal aeroplane. I agree absolutely about going the Rotax route for cheap personal flying.
G