Just noticed this thread while looking for something else on this topic... is he in yr 12 now, Lynette, looking to apply this autumn?
My DD is in yr 13, and now has a clutch of offers for EEE MEng courses, so we've been through some similar questions recently.( EEE because she doesn't know yet whether she's more interested in the Electrical or Electronic side - some of the courses diverge after year 1). One of her criteria was that the courses were IET accredited. (There was one which wasn't showing as such on their website because its due for renewal - if in doubt he can email the admissions tutor or ask at an open day)
Our impression is that at the places she was applying to, the norm is to go for MEng at the outset. I don't think there's any problem if students want to end after 3 years with a BEng (one prof specialising in high voltage said there's a shortage in that field, quite a few of his students get summer placements and get offered jobs by the company so don't do the 4th year). Students who enrol on BEng can usually change to MEng at the end of the first year subject to satisfactory results. However, it was unclear what the situation re loans is if you change from a 3 yr course to 4 year. Also, some will do a 'free insurance' offer - apply for the MEng and if they offer you a place on that you get a BEng offer a grade or two lower thrown in, which doesn't take up a UCAS slot.
I may have this wrong, but the case you cite they still need to get an MSc after the BEng to get accredited, so its still 4 years but its an additional student loan! We got the impression (at the places she was looking at) that most uk students do MEng(one degree, one loan), and that MScs were more commonly taken by overseas students.
Given which, there seemed to be little point applying for anything other than MEng. Details will of course vary depending on which unis he's interested in and what sort of expected grades. DD found going to quite a few open days helpful to get answers to these sort of questions.