Both aircraft benefit from being flown at FL60 plus which in the UK typically means airways.
For low level VFR would suggest the 182 uses 50 litres per hour, cruises at 120 -125 KTAS, and rents for around £230-250 per hour wet. Full throttle at FL80 and leaned it improves to 40-45 lph, and 130-135 KTAS.
Ensure you get a good checkout in particular technique for landing the 182 when at a forward CofG.
There are syndicate 6-300 in the London area and may be worth checking G-Info for London syndicate owned Sixes and approaching the owners.