Hi, Maureen!
Different varieties of Acer have different tolerances to cold, heat, drought, etc. Often those that are closest to the original species, such as common green seedlings, are the toughest. Named red and/or cutleaf varieties have been selected for leaf color and shape by the breeder, often with little attention to plant toughness. So a particular redleaf variety would have little trouble in Kyoto or San Francisco, where it was bred, but it will be more precarious in more marginal climates, such as the UK--especially during a monster winter! I'd give it another few weeks to sprout out, but if you have to replace it, look for a variety known to have survived this last winter. One other possible explanation for the survival of the green one is that it was planted in a different "microclimate", that is, a spot that had a little more shelter from the frost from surrounding plants or structures. Sometimes just a few feet in any direction can make a big difference! So yes, it probably was the severe winter. There have been lots of questions about dead Acers, Cordylines, palms, etc. on the forum--you're not alone.

i lost my garnet acer this year it was 4yrs old and very healthy, lost my red star cordy, tree fern and agapanthus in the last couple of years too! It seems the last 2 winters have been killers for my garden, even with fleece!