This question comes up from time to time and - as you found out yourself - nobody seems to have any first-hand experience of this. UK Visas insist (on the phone) that you need to get a new Family Permit, but the European Directive 2004/38 is very clear on this - your right to enter the UK with your wife is not dependent on the fulfillment of any administrative formality (like getting a family permit). As long as you can prove that she is married to you (hope your marriage certificate isn't still with the Home Office) and that you are a qualifying person (e.g. that you have a job in the UK), they are supposed to let you in - even without a passport (in theory).
I have also been told on the phone that one wouldn't get into trouble if both people travel together - but the woman I spoke to didn't really want to confirm this explicitly when I asked her to repeat it. ;-)
Since you don't really have any choice now, I would:

- make sure you have plenty of documentation that you are married to each other (the usual madness - joint bills, bank statements, etc), plus of course your marriage certificate
- proof that you yourself are employed in the UK
- a printout of the EU Directive (http://europa.eu/eur-lex/pri/en/oj/dat/2004/l_229/l_22920040629en00350048.pdf - article 5.4 is the important bit)
- a printout of the following document: http://homepage.mac.com/bncweb/guide_2004_38_ec_en.pdf
This used to be available on Franco Frattini's webpage (the Vice President of the European Commission -
http://ec.europa.eu/commission_barroso/frattini/doc/guide_2004_38_ec_en.pdf) but for some reason it is no longer there. (Still - it *was* written by the Vice President...)
- plus: don't make a seat reservation on a train that leaves one hour after your plane arrives...

Finally, once you have successfully re-entered the UK, please come back here and report on your experience as it will give others a better idea what actually happens...
Thanks!
Sebastian

 

Another thing you might want to try is to call immigration at Heathrow and ask them how they would deal with such a situation. This was once recommended to me by a UK Visas employee. I don't really know how helpful the answer would be, but if you get a verbal confirmation from a supervisor there (and his/her name), it might relieve you of some of your worries.
Secondly, it occurred to me that the first "barrier" that you'll have to cross may be the person who checks your passports in Mexico before you get on the plane to the UK/Europe. Luckily, Mexicans are not visa-nationals, so there's no reason why you should be refused to board, but if that person gets suspicious because of the expired residence card, it might take some explaining, too, as airlines are forced to transport "rejected" people back at their own expenses, I think.
Anyway - I don't think you should worry about this too much - but it is always good to be prepared. ;-)
Good luck!
Sebastian

 

Hi - how did it go?
Sebastian