The customs entry form does still exist on the Le Touquet web site at
www.aeroport-letouquet.com/customs
I'm also Denham-based and often pop over to Le Touquet. The simplest route is probably to simply follow the M25 round to Stapleford (taking care around Elstree on the way) and then just setting course from there to Cap Gris Nez. That'll keep you clear of Rochester, Headcorn and Lydd, taking you out over Folkestone for a short water crossing. If you want to practice using VORs on the way, use BNN - BPK - LAM - BNE, though check the NOTAMs to see if Boulogne is back in action now.
Denham will want you to fill in a GAR form. You can find a blank one in Word format
here.
You might also find
this basic guide useful.
I don't know how long you'll have there, but if you're going for lunch then do a search here for Le Touquet restaurants: it's a topic that has been quite well covered.
Finally, have fun!
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Edited to revise the link to the GAR form to point to the current version.
Edited again now that l'Escale has re-opened.