BSc before MBBS - worth it?

There is literally no benefit in doing that. You just limit yourself to at best a single medical school, in a transfer scheme that is likely to be 50x more competitive than applying for standard entry medicine - with about 250 students who all want to apply for 2-10 places (and so you normally need to be in the top 10 or so students - not 10%, top 10, in order to be considered). Also they require you meet all the exact same requirements for entry to the standard entry medicine course as well anyway normally, including academic requirements, UCAT, interview, etc.
You may as well take a gap year and reapply as then you have 4 medical schools you're applying to (and 4 chances), you're being considered for any of the hundreds of places on those courses and not just then ring fenced 2-10 places available for the transfer scheme, and you aren't having to also beat out 250 students to do better than all of them on first year degree level material while preparing for the UCAT and interview. You also waste a year of your SFE funding entitlement which means you no longer have the "gift year" available in case you had to retake a year later.
There is just no good reason to do that. It's essentially just a way for the uni to feed more people into the biomedical sciences course in question (as they don't tend to be that popular otherwise).

Thank you for your reply! I highly appreciate your opinion.

However, I do not think it possible to take another gap year when I am in one already. I was under the impression that Brighton and Sussex Medical school required undertaking the BMAT, but since BMAT is being cancelled from this year is it the UCAT now? As an international applicant, I think the chances of me getting in regardless are slim. I am just awaiting one more decision from UCAS out of the 4 medical schools I applied to this cycle.

I hadn't known that this programme limited you applying to only a single medical school. Assuming, I don’t enjoy my course as much in the span of a year then can I choose to apply to other medical schools in UK or Ireland afterwards?

I got the offer to pursue biomedical science at Brighton starting September, with the intention of switching to medicine at the earliest opportunity. Any advice on how I should proceed? The sheer volume of reddit posts advising to rather take a gap year than do a premed subject has me thinking if I am even walking in the right direction.

Hi!
Which country are you from if you don't mind me asking?
I'm a UK national and did a BSc in Biomedical Science here and then worked in laboratories (covid and NHS) and am now going into Graduate Medicine (same as the standard medicine course but you apply as a graduate and it is accelerated to 4 years rather than the standard 5).
From my understanding switching mid-course is difficult if at all possible. Obviously they will expect you to be a full graduate for graduate entry or in your final year, but I am not as sure for the 5 year course applications.
If switching mid-course is your goal, for future reference and clarity I would ASAP check and email all the med school admissions teams you look at applying to in the future to see if they allow mid-graduates to apply for both the standard 5 year (especially), but wouldn't be any harm in asking the 4 year graduate ones too. These can sometimes be subject to change so please keep that in mind for the future.
Knowing this from the medical schools you will not feel obliged to complete the full course unnecessarily (if you do not wish to complete).
If your end goal is medicine in the UK then you don't really need to do an undergrad. However, if you applied to somewhere like the USA then doing an undergraduate is required. Please also consider study duration and finances for this as this can be an expensive route, especially without scholarships.
If you did not get in as early as expected and had to work in the mean time, and wanted to work as a biomedical scientist specifically ensure your Brighton degree is IBMS accredited - if it is not you would have to get your degree assessed (at a cost to yourself) and possibly also perform top-up modules (also at a cost to yourself). Other work is fair game though.
Best of luck