Unfortunately the 'reasonable' bit is down to your own school's pay policy in which it they can and should define this in detail for progression to UPS.
Basically you must be 'highly competent in all the teaching standards' and your contribution to the whole school should be 'substantial and sustained'. Obviously this leads to wildly different interpretations.
But the criteria your school sets should not confuse UPS responsibilities with having a TLR - which is above and beyond what is expected of a class teacher, UPS or not. A TLR gives teachers the responsibility and accountability to 'lead, manage and develop a subject or teaching area' or 'manage pupil development across a curriculum area' and 'lead, develop and enhance the teaching practice and training of staff'.
The STPCD says that 'subject leadership' eg. classroom teachers, UPS or not, should mean that you 'contribute to curriculum development by sharing your expertise and advise on effective practice', but you do NOT have responsibility for a subject or the management and training of other staff.
So for UPS you can 'lead' a subject area by helping develop it and you can share your expertise in a way that contributes to the whole school (so leading CPD in a staff meeting, being observed by colleagues as you teach, helping develop curriculum plans etc) but you aren't responsible for training or results. Other ways to contribute are often things like leading School Council/Pupil Voice, mentoring an ECT or sometimes being a Year Group lead, or Key Stage lead, but again this doesn't make you responsible or accountable for results - that's a management or TLR role.
The whole thing is a bugbear of mine (Can you tell?
) as often the smaller the school the meatier and more 'work load' dependent your UPS expectations are, which is hugely unfair.
For my UPS1 requirements I 'lead' RE, PHSE and RSE, am the RWI and early reading/phonics lead and crunch the data for KS1 Pupil Premium/interventions. I think this is too much, but am currently in negotiations with my Head!