Hi,
I would suggest going to a book shop and picking up a copy of 'Trees'. It is a small book in the Collins Gem series. (It's about 3"x5") It easily fits in the pocket. It contains more trees than you will probably ever meet, certainly more than are on the moors. It has pictures of trees, leaves and their fruits, as well as descriptions.
Secondly, perhaps a trip to an arboretum? Trees there usually have a name tag with both the latin and common names on it. So you can actually see the tree rather than guessing if you have the right one. There is a small arboretum at Burrator, but I am unsure what it now contains. A much larger, national, arboretum is up at Westonbirt (Gloucestershire I think). You will need a whole day there, and still won't see all the trees!

I have been a few times, and they have some fascinating trees.
Finally, practice. Look at trees in towns and cities, along the streets and in parks,. You will start to recognise the more common ones - oak, ash etc - and then start to easily pick out what seems to be more unusual trees - that's when you need your book in your pocket! Once you start looking, you will actually see that there are quite a few uncommon trees located all around the place. Also look at the trees' bark and branches, as well as leaves. You may well need to recognise the tree in winter! And do not forget the firs and pines too
John.