Parents like me who wouldn't be able to afford £10, would more than likely but a card and a small thank you present like a box of chocolates.
Yes, I'd be pissed off at the 'veiled' threat, but I'd just ignore and do my own thing.
I have given that much three times, but the first time was DS2's reception teacher, who had been amazing and helped him so much (he has SN), and she was retiring.
The second time, his Y2 teacher was getting married, and again, she was excellent with him.
The third time, DD's form tutor had just had her baby, and DD wanted to get an outfit as her tutor had a girl, and DD loves baby girl clothes because she has 5 brothers all together but no sisters.
Mostly I get a box of chocolates though.