Small tree

All year interest you are looking at a conifer. Various different ones some stay relatively small others grow into monsters. Monkey puzzle gives interest all year round but they get huge too.

Otherwise look at Acers. Lovely spring foilage that turns either red or green then usually orange/yellow in the autumn. I would recommend Acer Palmatum 'Blood Good'.
I have one and the colours are fantastic. I also used to live in Oxford before I moved down south.

I also thought "Acer." Lovely feathery leaves, beautiful light green colour all year then spectacular red in autumn. Plus it stays small-ish.

Yes Acer would be good, and one in particular is Acer palmatum 'Winter Flame', a slower growing version of 'Senkaki', nice maple foliage in summer, good Autumn colour, and wonderful red stems in winter.

Yes, Acers are lovely,but have you considered choosing a large shrub instead of a tree. Then it won't grow too big and you can prune the lower branches to make a tree shape. Variegated holly would be of interest all year, or Amelanchier is a lovely small tree/ large shrub. It has creamy flowers in spring with bronzy new leaves , then green leaves and little red berries in June, then autumn colour. Yes, I'd choose Amelanchier.

I agree with variegated holly. It can be pruned to a nice tree shape. Acers are fine but they spend several months as bare twigs - also yellow-green columnar yew is slow growing and very nice all year round.

Celandine - I guess your not familiar with Acer 'Senkaki' and 'Winter Flame' then ?

I wanted a small tree for my front garden, several years ago, with lots of interest and I chose an amelanchier (as recommended above by volunteer). Birds absolutely love the berries on it as well

Itea virginica 'Henry's Garnet' would be a shrub that would more than rival Amelanchier, IMO, :o)

On this webside there are any conifer and anysize http://www.esveld.nl/engels.htm

Callicarpa bodinieri 'Profusion'