Yes, they are allowed extra time but extra time is not a universal panacea.
DD has very very low processing speeds and needs to concentrate hard. It is a long exam. Her experience was that she was exhausted at the end of normal time, and absolutely unable to do anything constructive in the extra. Not helped by test centre conditions, which again can vary widely. The air conditioning at her centre, in a Central London basement, had failed and it was the hottest day of that year. (Oh and a tip from a friend's son - take ear plugs, some centres are noisy.)
All applicants will have a range of strengths and weaknesses. DDs strengths were academic, interview and a good range and depth of extra curricular, including in health care/volunteering settings. She consistently performs poorly in UCAT type exams (CAT tests, 11+ and so on). Others might ace the UCAT but perhaps struggle at interview. Or struggle to get the offer grades.
The trick is in part to recognise where the issues might be, do your best to improve and to pick medical schools carefully, based on the emphasis they give to various proficiencies. Oddly having got through to interview she thinks her SEN may have helped her stand out. Medical schools like diversity amongst their intake, and probably like some who tick the SEN box but who are clearly capable academically. There are surprisingly few on her course who qualify for extra time.
(She came a long way from primary school where the Headmaster claimed she would not be able to cope with an academic or selective school, so the increasing focus on UCAT scores was a relatively small hurdle. The plan B would have been to look at the Republic of Ireland, pre-Brexit, where there is more emphasis on A level performance.)
DD worked for a well known family ski company as a chalet girl. It is a pity it completely put her off paeds, as she had plenty of interesting exposure to different family dynamics.