These things aren't real picky about oil, fuel or being stored wet or dry.
Mixing oil is of no concern. I'd prefer the leftover 5W-40 but the amount this engine needs is less than a quart on a drain and the UK is typically not that hot a climate anyway.
Don't overthink this as these are very durable engines that suffer abuse without flinching.
Just ensure that there's always an adequate quantity of oil in the sump.