A major point here is that having a career backup is a good thing, but in many cases a degree is not that. Train as a lifeguard, or a chef, or a plumber - all of which will be quicker and cheaper than getting a degree, and likely to get you a job faster when you need one.
A 4 year engineering degree, at £9k.pa tuition, plus living expenses is one hell of an investment in yourself. There is, in my view, only one reason to do it - a passion for engineering and likely interest in taking that career path.
Also do a degree, any degree, then switch to pilot training - and you will find yourself 2 years later with your degree skills rusty and potentially in competition against people who have just graduated and are a lot sharper than you are now. As a backup plan, it is an absolutely lousy one. Also bear in mind that an MEng graduate is applying for trainee positions - it's another 4 years to get your CEng.
There is a bit of an exception - if you are dead-set on an aviation career, but are more relaxed about *what* aviation career, then the combo of an aero-eng degree and licences, may open a lot of doors to you. But let's not pretend that's a cheap or easy option - 4 years to get the degree plus 2 years to get a CPL, leaves you probably £130k in debt and still at the bottom of either career ladder.
I would say pick one and take it seriously - then consider doing the other in your spare time whilst working, either an engineering degree with the OU whilst flying or job hunting, or modular and PAYG whilst working as an engineer. But studying an aero-eng degree, to then go straight into pilot training - that strikes me as very poor planning indeed in the modern world.