It's presumably (ultimately) funded in the same way that other ITV content is; i.e. through advertising during the breaks and at either end of the show.
Paying to improve someone's garden isn't a major item of expenditure when compared to other TV production costs. An hour-long 'reality TV' show typically costs £50,000 to make, whereas sitcoms cost around £400,000 per hour, mid-budget drama costs about £500,000 per hour and blockbuster costume dramas cost about one million pounds per hour. So paying for some plants, decking, water features or whatever isn't going to knock too big a hole in the advertising revenue!