There's a lot of them after you get past Wiltshire going east on the M4.. they look a lot like the cherry/apple trees in my garden. They are on the edges of woods and along the verges of the motorway. Some of them are very tall, taller than ornamental trees I see and I noticed some were quite smooth of bark on the trunk, again unlike (from memory) our 20 year old tree we used to have.

Are they likely to be:

a) garden escapees
b) wild cherries?
c) crabapples?
d) something else?

b

Wild cherries common then?

d?

Blackthorn is also in flower at the moment, could be a whole host of things

Not blackthorn/hawthorn. We are talking big flowers either white or with varying degrees of pinkish hue that look very similar to ornamental cherries or apple/pear trees.

Prunus avium, the wild cherry, is native to the UK so may well be on roadsides.

Could possibly be Cherry Plum trees? I have one in my garden which is flowering at the moment. The blossom is white but I think they can also get pink blossom on them too. The bark is quite smooth with the odd spikey bit, and it's quite a big tree.