Now, I don't have my car yet (

) so I can only make the assumption that the undertray bulge issue is caused by the mounting holes being positioned wrongly on the car frame or the undertray (whichever). The installer probably then bodged it rather than risking highlighting the problem and losing his job (my opinion). If that is the case, then I rather think the best solution is one I've seen suggested before in various posting:
- Remove the offending undertray
- Observe and measure (using string, tape measure, cardboard, laser, radar or whatever you have to hand) where the actual bolt holes are on the car frame.
- Make sure the undertray is flat (boiling water, stand on it, whatever works)
- Match the measurements of the car frame bolt holes to the holes in the undertray and drill new holes/elongate existing holes to suit.
- Bolt the undertray back into place with repair washers wherever a hole has had to be modified.
I realise that this task is not something every owner could or should be expected to do, or maybe is even capable of, but I doubt it's a huge job if the bulge concerns you.
I suggest it should really be performed as part of the PDI, pending a proper MG fix (I.e. a new undertray with the boltholes in the right places), when they install the new breather



).
It'd give the dealer ample opportunity to spot any oil leak/spillage/splash etc. as well.