Like it or not Jonny Foreigner and their religious beliefs have got us over a barrel, if they refuse to do pork ect and get the sack they will win the day in court for wrongful dismissal and will be laughing all the way to the bank.
Doesn't seem a very insidious threat to me. On the one side someone might have to wait a bit longer to buy some champagne (as opposed to people who get refused sale every day) and yet on the other end of the scale which you mention they may lose their job, find it much harder to find future employment, have to go to an employment tribunal and then finally get some settlement (if they win and if they even go in the first place).
Under the old rules for unfair dismissal a worker could theoretically get more than a year's salary, although this was extremely rare and capped at about seventy grand. Even before the new rules for awards came in a lot of cases the awards were three or four thousand.pounds.
Taking a ball park guess at a payout under the best possible conditions. A worker in M&S is likely to be on about £7 an hour (probably a bit less). They are probably not that likely to be full time, so let's call that 25 hours (probably generous in today's climate). So the maximum payout they could get would be around £9000. Note that awards in real life are very much lower than this.
Sounds more like Barry the Bulldog of Big Business has Johnny Foreigner over a barrel to me.