I'm thinking of treating myself to first class; I've waited a lifetime to do this trip and in any case, it might just be worth it to make the inevitable journeys in crosscountry a little less uncomfortable, with at least a bit better chance of a seat.
Where you are relatively certain you will use a particular service I would still recommended reserving a seat.
Especially on LNER as they generally have high occupancy of first class, LNER even have a free reservation tool on their website where you can pick the exact seat you want.
Cross Country can get very busy in first class and Avanti on weekends only have 18 first class seats in carriage K on Pendolino services.
and assuming I go for the 1st class option, a trip on the Marches line in a Mk5.
Slight correction, Transport for Wales use Mk4 not Mk5.
If you do mean Mk5 specifically Chiltern Railways operate Mk5A carriages on some Birmingham Moor Street to London Marylebone services (these have good first class seats but no catering), otherwise its the Caledonian Sleeper for Mk5.
I'm considering FW-Mallaig and Inverness-Wick; both of which I've not done. I'm pretty familiar with everything in England west of Swindon from my previous forays with the FOSW rover, so will likely give that a miss this time.
The Mallaig and Wick are both great journeys but considering there going to realistically take a day each out of your rover may not be the best value option on an ALR ticket, especially in first class as these are all standard class only.
Also consider at this time of year it will start getting darker earlier in the evenings so you may be best off waiting to do these routes in the summer months.
Apart from LNER/Avanti, are there any decent first class meal offerings?
LNER (especially Signature menu services) and Avanti have the best complimentary meal options.
Transport for Wales (as you mentioned earlier) have very good first class dining including both Breakfast and full 3 course meals but these are paid extra, though are definitely worth it in my opinion. You get tea, coffee, water, orange juice, shortbread, crisps as complimentary offerings as well.
GWR Pullman Dining is another paid extra (and quite expensive compared to TfW) but is worth trying at least once if you have not done it before. Anyone can dine if space permits but first class ticket holders can reserve a place before travelling.
All other GWR services do not offer full meal options but do have a good range of snacks and offer pastries/porridge on weekday mornings.
TransPennine Express on their Coast to Coast (Liverpool to Newcastle) and West Coast Kitchen (Manchester to Glasgow/Edinburgh) have decent complimentary meal offerings. Though do check timings as not all Coast to Coast services have catering and some early morning Glasgow West Coast services do not have the full offer.
While not a substantial meal I find the Hull Trains morning breakfast rolls quite good. Hull Trains rest of day offering while basic (wraps/sandwiches) has always been consistent and I have enjoyed using it.
Cross Country can be good but it is very inconsistent as to what offering you will get, do not rely on what is advertised in the timetable. I have found it difficult to get their full meal offering on many routes (for me the most reliable has been Edinburgh to Leeds section of the route) and even then the XC first class offering I find to be to a much lower standard (in terms of options and availability) compared to Avanti/LNER complimentary offerings.
East Midlands Railway offer breakfast rolls/sandwiches on weekdays and snackboxes on weekends/some weekday services, I would not consider these a substantial meal but if you are using other services can be a good fill in option between more significant offerings.
If you were to use the Caledonian Sleeper I find their food offering decent, though it is a paid extra and relatively expensive for what you get (breakfast is included in Club/Double).
For completeness other operators that have complimentary food/drink offerings but that I would not consider full meals are:
Grand Central (tea/coffee/water/snacks)
Greater Anglia most Norwich to London weekdays only (tea/coffee/water/biscuits)
ScotRail most Intercity routes but not all services (tea/coffee/water/biscuits)