Torn Between Courses

Yep, the masters funding is separate from UG funding. Unless you do a 4 year undergraduate masters degree (e.g. MPhys, MChem, etc) you shouldn't have any issues there.

So as harrysbar says, you get a "gift year" from SFE, as your total entitlement (which you can calculate in any given year) is: length of degree + 1 year - number of years of prior study at HE level. So doing one year somewhere else would use up the "gift year" and you would still have enough for full funding for any degree of any length.

However, you would not have any funding available if you had to resit a year, or decided to change course again, in which case you would have to self fund a year with only a maintenance loan and no tuition fee loan (and you would still need to pay tuition to your uni). Additionally if there is a situation where your total remaining entilement is less than how many years you have remaining to study, you have to self fund the very first year you are going back to uni where you don't have funding left. You can't get funding for all years up until final year and then not be funded that year and self fund yourself then, which means you can't have several years to potentially save up money before having to self fund, without actually withdrawing from the course or agreeing with your uni to take a year out (various terms for this, intermitting, interruption of studies, etc).

So it is theoretically possible but I'm not sure realistically that good of an idea. I think it would be better to take a gap year and spend some time exploring each subject so you can make an application and fully commit to the subject. Bear in mind many if not most students studying both CS and psychology at uni won't have any specific background in those subject areas, but will often have studied related subjects (sometimes by requirement) such as maths for CS, or biology and/or other social sciences for psychology.