I've found that combining traditions works well for me. While my DF's parents are both departed, he still enjoys the full-on Sunday tea. That's great, fine and dandy, but I have added my own things to it...that way I can enjoy a taste of home. Some things I've added are sweets, others are savory yummies--which certainly helps lessen the sugar-round-up.
Not sure if you want "suggestions" or not--but maybe offer your MIL some help...tell her that, with her blessings, you would like to introduce a few "American-isms" to the tea, while keeping the English tradition that she holds so dear. She may appreciate the help and who knows, the new savory treats can make the day a little more special...
Of course, if there is nothing to be done to alter the situation, look at it this way: Your MIL is in her 70s. She is set in her ways and does these things out of love and care (one would assume). At some point in time, she will be gone from this world, just a memory to all who loved her. Think of the fond memories you are creating for you and your hubby/kids as well as the pleasure it is giving an old woman in her twilight years.
Just a thought...