some of the Sorbus group make nice small trees, Rowan can be pollarded to good effect and fit the size you want. Some of the named variety of birch also.

My first thought was Whitebeam - it will eventually grow much higher than 5m but is quite amenable to being kept to size (from what I've seen). It is native to Southern England and has winter attraction in berries.
Another thought would be small fruit trees or some of the decorative cherry trees or flowering almond.

Its the under planting that could be a problem ........

Thank you to Seaburngirl, Urbanite and Pamg for your responses. I will certainly look at Whitebeam and some Sorbus. I have seen this work before and the Hebe makes a lovely rounded mound at the base. I realise the trees will need looking after, watering etc. as they get established, so hope the underplanting is not an issue. I did not know you could pollard a Rowan, which I love. So thank you all.

the rowan outside my mums house was pollarded by the council. it was stunning.

Why should the underplanting be a problem, Pam?

As long as the tree is allowed to establish itself - ie prepare the area properly and don't plant the hebe in the first year - I can't see that it's any different from anything growing at the foot of a tree. A baby hebe might struggle against a full grown tree but these will grow up together and will adapt to each other. I'd only let the tree establish itself a little because of the relative cost of replacement in the case of a failure.

Can you grow dogwoods in the UK? Check out the dogwoods. They are understory trees, available in various colors and attributes. They put on quite a show and never need pruning.

The cornus family thrive in the uk, but you do have to prune them so you get the best stem colour if not they will get very tall, leggy and the color will be not as vibrant.

Just a couple of ideas here, a small standard Salix pink flamingo can be rounded and looks great under planted with a varied choice of plants, in winter it's a joy to behold with the tips a lovely shade of pink, and this is what I have done over in France , birch trees planted eight years ago as four year splings, over the years I have taken the lower branches off leaving a long slender white trunk, the tops of the trees have been rounded and I have under planted these with perennials of choice and some in a see of cornus with midwinter fire being the best, the effect is breathtaking more so in the winter with all that color going on, in spring you can add some snowdrops between the cornus as well as other spring bulbs.

Thank you Julien too - have looked the Salix up and love it. I haven't used this site before but am overwhelmed by other people's help! Amazing.

Light and shade under trees is what was in my mind, spring flowers do well as the trees are usually deciduous and the light levels good, summer plants may struggle for light and moisture.....
All depends on the conditions I guess......