Priority number one has to be to find out what you a e dealing with. The low sugar thing could be either either end of a spectrum, from avoids desserts and sweets and chocs, to carb counts for every meal.
Do you have a particular meal you usually serve on Boxing Day?
In this days after for main meals I would serve:
Chilli (meat or veg) with rice, sour cream, guacamole, salsa and tortilla chips.
Chicken, chorizo, garlic, potatoes, red onion, peppers, tray bake.
Cauliflower, chickpea and red pepper curry with rice, naan, popadoms and mango chutney.
Tuna pasta bake (Lacto free cheese and breadcrumbs on top, tomato sauce as base) or chicken, bacon and leek pasta bake, with steamed broccoli.
Sausage casserole, baked potatoes, peas and carrots.
Most of the above are on the whole naturally lactose free (minus sour cream with chilli but obviously that’s added by the guests themselves) and if your low sugar guest happened to be low carb, easy to omit the carb for that guest (although not the pasta bake.) Also all fairly easy to cook for large numbers without being hugely expensive. Offering a few side dishes helps to cater for all tastes.
As others have said, keep breakfast and lunches simple.
Porridge, cereal, toast, fresh fruit, tea, coffee and juice.
If you want to offer a little more variety, offer different bread options of bread products. Crumpets one day, bagels another (maybe with cream cheese and smoked salmon if you want to offer something a little more special.) part baked baguettes another day, the frozen breakfast pastries from Waitrose another.
I would avoid offering a full English but may do a pan of scrambled eggs one day and stick a tray of sausages in the oven another so people could make themselves a sausage sandwich.
For lunch, again different types of breads mix things up a bit, pitas one day, wraps another, baguettes another. Then just a selection of sandwich fillings, salad bits etc. Cheese, pates, crackers, crudités, pickles etc another day. Only hot lunch bits I might consider would be a big pan of soup or putting a quiche in the oven (either shop bought or made in advance and frozen.)