Hocus - you need a post 16 evaluation to qualify for DSA. If you qualify for DSA then that triggers the full range of support etc (much of which won't be cash in hand but may involve study support, adjustments etc) from wherever you go. Not all institutions make qualifying for DSA their benchmark (at least not every one that we have looked at (which is obviously a tiny number) mention DSA as their trigger for support, but most of the ones that we looked at do make DSA a hard and fast triggering qualification.
Not all school SENCOs are up to speed on this.
Obviously using a keyboard for exams might not in and of itself mean that someone qualifies for DSA since custom and practice is the key trigger for that.
Uhtred I would agree with this and I think I said that we were just going through this at the moment. All I can say is that at Ds's particular university they want you to go through the DSA process, and it appears to be the "trigger". I agree this does not mean "funds" in the sense of actual money. As I understand it we have to do this for DS (at his particular university) to qualify for using a computer in exams ( as will be further qualified by the university - he has done it for GCSE and A Level - but the uni will want to check for themselves.) It's not money but it seems to be the way my son's uni check they can give some adjustment - I hope that makes sense. My post was probably not detailed enough but just trying to give some experience as to how it is working with DS. If other unis don't want the DSA process - well that's a bonus in my book.
I did say someone else would know better than me 
In which case LIZS - please read Uhtred's post and make sure you know how it works for the particular place your son wants to go to. Sorry - just giving our experience.
Also Uhtred I would agree re school SENCO. When DS needed to use a laptop for A levels, we checked with School senco that this was agreed with exam board. . Then check the box on the UCAS form. Only after that when your DC has an offer from where they went to go , then - as you say - check with the particular university how they do it.