Never seen a trolley with any form of interlock. If the token is only 1 pound, it's hard to see why an interlock would be justified. Perhaps if you were talking a tenner or more it might make sense – but what kind of place would require that?
At Aldi the other day and the assistant lent me her key (from a corned beef tin). rather then give me change. Once I'd released the trolley, she got it back.
From a design point of view, if you insist on making something, the token should be oval so that you can get your keys off after releasing the trolley. Especially if you are not obliged to return the trolley to the trolley park.
The simplest solution is to use your car key to release the trolley. Once you have got the knack it takes a split second. No need for anything fancy then!
Merry