You don't need to get central A/C or mini splits. You can use an efficient Window unit (not those single ducted portable units that are just barely better than nothing. if portable do dual ducted for efficiency) Those window units are available at Walmart in the US for a couple hundred apiece. Presumably hypermarts like Carrefour would carry them or some places that serve home improvement.
Efficient window unit?
Best of the best is about 15-16 SEER
That's entry level central HVAC efficiency
Minisplits are far higher, 20+
For some reason it's very hard to find window units for sale in the UK, single-duct portables are the only thing available for cheap (although it's a fairly easy mod to convert one to dual-duct).