As you're in the UK, I would take a trip to Aldi and buy some custard (45p 1/2 litre), porridge oats 79p a bag (soak in hot water makes great gunge). Aldi even had marshmallow fluff at 79p a few weeks ago (which i love, can be thinned with hot water and is super sticky) but I suspect the stock has now gone. You can get cheap shaving foam in Tesco and Sainsbury's 50p a can, not only great to make pies but also mixed well with warm water makes a reasonable gunge. All of this washes off easily.
That's for starters anyway...
Don't forget a bucket of shaving foam over your head... LOL
Hi ! A pot of creme fraiche when mixed with custard also makes a think gloopy pie mix. A small pot makes a few pies : Aldi does cheap creme fraiche. Enjoy !