The official description for this sandwich is:
Rare roast British beef with horseradish mayonnaise and lettuce on white breadThis sandwich has suffered from a classic case of overmayonation; the quality of the beef (as good as you would expect from M&S) is sadly masked by an overpowering layer of creamy mayonnaise, making it difficult to enjoy the other ingredients. It doesn't help that this is supposed to be 'horseradish mayonnaise', but there is only the merest hint of horseradish – it is so mild, it may as well not really be there at all.
Unfortunately, this results in a sandwich which is bland and unsatisfying. Personally, I would prefer just horseradish sauce, not a specially flavoured mayonnaise, and I would like it to have a bit more bit to it.
I would not buy this sandwich again.