I wondered this the other week when reading the back of one of those tickets; at that point it was just curiosity as the opportunity was a very rare treat - however in reading up now (following your question) I note
http://www.nationalrail.co.uk/times_fares/ticket_types/46544.aspxConditions of use
You must travel in the class of accommodation shown on your ticket.
http://www.nationalrail.co.uk/static/documents/content/NRCOC.pdfYour obligations
You must:
^ Travel in the right train(s) and the right part of the train(s) - otherwise you could be charged an excess or Penalty Fare. You may only change from standard class to first class areas if a member of staff in that train gives permission or you have paid the appropriate additional fare.
(my emphasis - then the following sentence muddies the waters and reads just one way, remind you not to be upsitting, but it does not exclude downsitting)
and
4b
You may be liable to pay a Penalty Fare if:
(b) you travel in a class of accommodation for which the ticket is not valid;
So it looks like you are required to use the first class accommodation if it's available (even to the extent of standing in that part of the train if full when there are standard seats??) and failure to do so isn't just a misdemeanour, but leaves you liable for a penalty fare in those areas. The wording in three separate places is consistent in pointing to this and it does NOT say "you must have a ticket for at least a class as high as you are travelling in" in any of the places.
I have to say I don't think you would have a problem, although staff may check that you understood that you were using a lesser service that ticketed for, and perhaps even wonder if you were pulling some sort of scam / fraud by downgrading. I can't myself spot any obvious way of making illicit gain by sitting in a standard seat on a first ticket, but there are some bright people out there who find all sorts of odd loopholes.