But it is legal. The obligation is on the reviewer to highlight they've been paid for a review, otherwise they are illegally misleading their readers. Whether that's moral is another thing.
> Don’t commission third parties to write fake reviews – you may be liable for their actions
https://www.gov.uk/government/publications/online-reviews-an...
With proper disclosure there may be some leeway, but I wouldn't just assume paying someone for a fake review is legal. UK advertising laws can be very strict in some cases (and rightfully so, in my opinion).