Following on from my post about the back stage tours offered by the National Theatre, my next recommendation is a tour of Angels Costumes. Angels is one of the largest (if not the largest) and well-known costume suppliers to theatre and film productions and offers tours of its warehouse in West Hendon.
The tour lasts approximately two hours and costs £20. It is a fascinating business, ranging from the creation of customised costumes for films such as Rocket Man to curating period outfits for series such as The Crown drawn from the 8 miles worth of clothes hanging in the warehouse.
If you like film or theatre, you should definitely put this one on your list. Be warned, booking is low-tech, you will have to email them for their list of upcoming tours. The website has all the details you need.