In some cases you can install what is called a "coin mech" instead of a coin comparitor. The coin mech is a coin acceptor that isn't connected to wiring, it is completely mechanical. It takes the inserted coin and verifies it, then the coin drops thru the bottom of the coin mech. Often the coin mech will work just fine and is lower cost than a coin comparitor. You would just get a coin mech that works with the size or type of coin you want the machine to use.
If you want to use a coin comparitor then you will need to get the proper one that works for your particular machine. Coin comparitors come in many different models and the voltage they operate from varies. The nice thing about a coin comparitor is that you can insert a wide range of different coins into the coin comparitor as a reference for what coin that you want the slot machine to use.
I think the UK 2 pence coin is about 26mm diameter in size. This is larger than a US quarter and slightly smaller than the current US dollar coin (older Eisenhower dollar coins were much larger at 38mm).
If you can post a photo of the inside of the door then others here can give advice on what your options are.