Never been asked for validation for one ....best one currently is TESCO (£20pp)...presumably inconjunction with shopping at Tesco ????
These codes are readily available on various discussion boards so I am assuming they are fairly widely used
I'm not sure quite how last minute you were thinking of booking but I've noticed on the website that mostly these codes can't be applied to the very last minute 'give away' deals (ie flights costing under £100 or so).

I used "divers" £15pp discount on a late, 2 days before flight booking from Birmingham-Malaga a few weeks ago, worked out at £70pp total, sure "tesco" would have worked too but forgot about it in my excitement !!

Check the "bargain priced flights" on the home page but dont click on that link, instead do a normal search, using the dates etc., from the home page and enter the code, it should then knock the £20 off.

I have used codes with XL many a time and have never been challenged.

Flights Gatwick-Faro for the 31st Aug are coming up as -£1, but are actually £39 with tax, fuel supps and tod charges, still a bargain imo.