Not consistently.
Some train companies do produce posters, but the loadings are probably extremely inconsistent and highly changeable at the moment.
LNER will let you see which seats are available to reserve, at the time of booking. Trainsplit does the same thing for all relevant train companies that offer reservations.
Your best bet is probably to post your journey requirements here, and people with experience may be able to advise.
Note that social distancing isn't required; it's merely a guideline, and the mandating of masks recognises that it is not always possible:
https://www.who.int/emergencies/dis...wers-hub/q-a-detail/q-a-on-covid-19-and-masks
for areas of widespread transmission, with limited capacity for implementing control measures and especially in settings where physical distancing of at least 1 metre is not possible – such as on public transport, in shops or in other confined or crowded environments – WHO advises governments to encourage the general public to use non-medical fabric masks.
In general, as a very rough guide, morning "peak" trains tend to be the least busy, with the first "off peak" trains in the morning generally the busiest.
On some long distance routes, so-called "peak" trains can give you an entire coach to yourself (e.g. EMR or Avanti out of London); business and long distance commuting traffic has been absolutely decimated and the rail industry doesn't seem to want to regain this traffic. In contrast, on shorter distance off peak services, social distancing can be difficult (though as stated above it's not mandatory anyway).
