I think that there's a reasonable choice if you ditch the requirement for a second passenger, or ditch the aerobatic requirement.
Both? A very limited choice, and still less under £40k.
Maule 7? Ex-Swedish Bulldog with a back seat? Lycoming engined Auster?
I'd say ditch the second passenger requirement and stick to 2-seaters - you'll get far more aeroplane for your money, and can always rent a C172 or PA28 occasionally to take that second pax.
G