In theory, you can take your new MG anywhere to get it serviced, provided they follow the manufacturer's service schedule and use OEM parts. You could even do it yourself (which is realistic with EVs needing mainly inspection).
But, in practice this only applies to a new car warranty, not any extended or other warranty and if you have bought on finance (as most do), it may well require you to take it to MG in your finance T&Cs, in order to get your MGFV.
And, also in practice, you are probably not going to have whatever expensive proprietary equipment is required to do things like software updates.
And, then if there's a problem, the onus will be on YOU to prove that your independent servicing met MG's standard and did not cause the issue.
So, probably better to just use the dealer. If you can afford a new car, you can probably afford the (admittedly overpriced) servicing.