I've been a class rep for both my kids for 2 years. We always suggest £10 for teacher and 5 for TA. its voluntary, but thats what most of them have been giving to be honest. For the first time, we're using www.giftwaffles.com
to collect the money online and get the vouchers automatically... it's made my life so much easier as i don't need to chase parents for cash at the school (cringe!) or worry promises of cash that never come through..
But yes I agree, class reps should give you guidance!