Similarly to when to do medicine, you need to weight up the pro's and cons of doing a second degree immediately vs. delaying by a few years.
If you complete both foundation years prior to starting dental school you have the advantage of full GMC registration and more experience which makes you more employable as a locum SHO.
Taking this further, if you complete core surgical training (CST), you are even more desirable. You will also have completed more courses, audits, research etc to pass your annual reviews which will also help with getting in to dental school.
With many hospitals and locum agencies capping locum rates, if you can work at CT2/ST2 level you will earn more than FY2 level as rates are calculated as a percentage increase above base rate. You might have some experience of acting up as a registrar as well which could help you get second on locum shifts which pay better and allow you to get some non-resident on call sleep if in the right specialty. Finally, you should get your MRCS exam done before starting dental school if you have done CST.
There are a few deaneries who have opted for a run through training path. That means rather than doing CT1&2, you do ST1&2 instead then automatically carry on to ST3 without another interview. If you do your second degree after FY2 this option is open to you.
Some would rather go somewhere with guaranteed work for 4 years over a 3 year course with sporadic work. Your personal financial situation will obviously come into play here.
Bare in mind it's perhaps not necessarily true to say there's more scope for part time work on a 4 year course as the first year of a 4 year grad entry course is often very intense and in a smaller group, i.e. it's noticeable if you're not around because you're working!
Some Universities are very supportive and help with work. Cambridge runs a 4 years course with a handful of dentists on. Again it is slightly more intense but permanent work is available at Addenbrookes and the uni are excellent with supporting second degree students.
Some advise go for the shortest route possible. Remember a three year course is more like 2 years 8 months at places like Bart's!
Main negatives of an extra year is obviously an extra year of paying out £9k, an extra year of locum jobs and perhaps varying income, an extra year of being a student. 3 year medicine is sometimes a bit of a blag in some people's experience; you don't always have the full background knowledge but you just find time to learn what you don't know.
Some 3 year courses may be less well organised, and to get the most out of it/pass exams you will need to be proactive. You will sort out the balance of locum work vs uni work when you start based on what seems like the next hurdle to jump (bills vs. assessments)!
Go for whatever you get. Better to do a 4 year than turn it down in the hope of a 3 the following year and end up with nothing at all.
Don't worry about taking a year longer though if you do get a 4 year; OMFS is a long journey so just enjoy it! If you are that keen on OMFS take what you are given.