In most cases I would not advise taking a masters to strengthen a medicine application, in your case though there are some clear benefits.
In addition to Swansea becoming an option, Warwick and Kings (provided your previous degree was of science) would also be worth a look. However, some universities would not consider the MSc until it is completed or they may not accept the application entirely as the MSc tends to finish in Sept, thus coinciding with the GEM start dates. Certainly email universities to find out where you stand on this.
I wouldn't assume you have to take an MSc in the same subject as before. People often use an MSc as a means of retraining. Just find something you enjoy.
An MSc course is quite demanding. Mine only required me to attend 2 days of the week, 9am to 6pm, lectures and labs. The workload outside of direct contact was quite high, generally at this level your independent work is expected to be greater than your direct contact. Work included essays, presentations and journal clubs, with exams forming the bulk of the taught marks.
The final semester's project is very time consuming, there were days when I was in the lab 8:30am to 10pm. You usually do not have much time to gather results so you're running a lot of experiments concurrently. This is something you will not
Besides the cost of fees and living costs, there is also the non-pecuniary costs; working for more than 20hrs/wk is probably not feasable, working during the project piece is certainly not feasable. Thus you would struggle to save for the GEM course that year and also your time for interview preparation would be limited.
As far as I'm aware PG funding will not be available until 2016, though I have not looked in to this since before Christmas.
Just consider these before you commit to a MSc to bolster your medicine application. Though these are my experiences and courses will vary somewhat, there is a general consensus that it requires a heavy time investment. I and others have likened it to completing years 2 and 3 of a BSc in one year.
If I were you, I would inquire at the universities in question re. completing an MSc during the application year. I had heard SGUL simply do not accept such applications, but another tells me they have a deferred offer from SGUL because of the MSc.
Wait until you receive your GAMSAT March results before you make any decisions for the next year.
Consider sitting the UKCAT if KCL and Warwick accept completing the MSc in the year you apply.
I did not apply for my MSc until the July before starting in Sept, one of my cohort applied the day before induction began. So you have oodles to time to decide as yet.
It's difficult not to become so easily attached to a particular university. Nottingham seems to attract quite a few individuals, it certainly did me. However, given my performance at Notts interview, I expect I will be taking up my offer at Swansea.
It's a not easy to accept, but it's easier when you realise you will still become a doctor. The experience may be different but you will still enjoy it and meet great people.
Swansea is a bit easier because it is pretty high up in my estimations any way, but the point is even if you don't make it to your top choice there will be one very similar and hopefully you'll make it in there.