If you're taking a whole trolley full through the basket only/small number of items queue, then you are abusing the system, whether you have a disability or not. If your trolley only contains a few things, then reasonable people shouldn't mind one way or the other. If the staff and system are reasonable, that should help engender reasonableness in customers too. I know some places use the phrase 'Express till', perhaps to avoid this whole conveyance and number issue.