As I read the OP, the question is about guiding the MIL to buy suitable stuff, rather than the OP planning to cook from scratch during her stay.
We eat a fair bit of Quorn, and the DC find it very acceptable. I do know about the processed/allergic issues, but for various diplomatic reasons I don't interfere (DH is SAHD and does all cooking and meal planning), and it works for us. We don't like Quorn sausages.
OP, you and your MIL will easily find a big selection of ready-made food products in all decent-sized supermarkets - pies, sausages, burgers, and similar; filled pastas; pizzas. etc. As one poster said above, the issue may be getting enough protein. It's a shame your DS does not enjoy eggs.
you might suggest she gets Quorn mince and makes a tomato/mince sauce for pasta? (a version of spaghetti bolognese). If she cooks with mince she will be on familiar ground, but if she doesn't already use lentils and pulses, she might find it a bit of a challenge.
Other good brands are Linda McCartney or Cauldron foods.
perhaps you can go shopping with her soon after you arrive, to see what will work for you all.
Fundamental mainstays for many British children, whether or not they are vegetarians, are tinned baked beans in tomato sauce on toast (in our house we buy supermarket own brand, Heinz are terribly sweet); and hummous, which is eaten by the tonne.