B and C need to grow the fuck up. They are adults, and it is THEIR responsibility to get themselves to work, nobody else's; and certainly not A's.
Since they flatshare, I presume A & B discussed the getting to work before B took the job, but B is still abdicating responsibility for themselves by complaining about cost. They should be comparing the cost with what else they'd have to do if A didn't give them a lift.
C sounds like a bit of a user. I'd wonder if she contributed to costs at all when her boyfriend gave her a lift.
If I were A, I think I'd just spell it out. 'I am doing you a favour by giving you a lift, your moaning about contributing to the costs tells me you don't appreciate that it's a favour. I find your sense of entitlement upsetting. I think you should make your own arrangements from date X.'
They managed to arrange lifts before, they can do it again. I appreciate he doesn't want to fall out with B, but neither should let himself he be pushed around by B; I suspect B is playing on A's preference to not fall out. If they are genuine friends, B should get that he's been a cheeky fucker, and this is the consequence. If he can't accept that gracefully, he's not really A's friend.
Frankly, it would be worth £20/week to me to not feel put upon by these two.