I think you have to find what works for you. Some people like contactless because if they have the right account, it tells you what you have spent, so if your app is telling you you're spending £70 pm in Starbucks or whatever, it's there right in front of you, rather than just a load of cash withdrawals with no further information.
Others think that tapping around here there and everywhere is too easy and causes overspending.
Others don't like cash because once it's in your purse, or you break a note, it just runs through your fingers like water.
But for the OP, I'd look at what that £100 pw is going on and see what changes can be made to reduce this if it's unaffordable, or you'd rather use the money for something more worthwhile, like a house deposit.
It might be as simple as keeping pop, crisps and sandwich materials or diluting juice and refillable bottles at home, taking a packed lunch when you go out, because these things cost much much less like this than they do when bought out, even at cheaper places like Greggs or supermarket meal deals, and lets not even think about the cost at service stations, nice cafes, theme parks or places like Pret 
.