First I think it's a little funny a gardener from the Netherlands chose to come here for bulb suggestions, I would think you could step outside and find something around the corner!
You seem to have an excellent starting point already, are you just looking for plant suggestions? I would think that for a smaller space such as you have you may need to plant more annuals and less bulbs. The begonias should work well but many of the other bulbs will only bloom for a shorter time and then leave you at best with semi-attractive foliage or worse, yellowing unattractive leaves.
I would try to limit the colors so there's not too much going on, stick with the colors of the begonias and maybe something darker for contrast. Will the begonias be tall enough to show over the boxwood?
I would think a few snowdrops for early spring and a couple bunches of tulips would be worth the yellow leaves, after that plant the summer color.
Hope some of this was mildly helpful, any other thoughts?

Are you aiming to do bedding or permanent plantings? If you want flowers all year round in such a small space you'll have to do bedding and change the plants several times a year. You wouldn't be able to have constant flowers using only bulbs/corms/rhizomes. If it is just colour you want all year you could do it with some permanent background plants plus bedding.

At this time of year you can be putting in forget-me-nots, polyanthus, wallflowers, violas, grasses, double Bellis perennis etc. underplanted with bulbs. Any local garden centre will have stacks of choices for winter bedding.

Regarding your begonia choices, bear in mind that all the flowers in those beds will be seen from above. The begonias you mention look better seen from the side, in a raised pot for example, as they tend to hang down slightly.

Remember too that box edging is a haven for snails and slugs so you may need to protect your flowers. They adore bulb flowers and frequently eat my snowdrop, daffodil and hyacinth blossoms before they've even opened.

But as kato_b says you will probably get more relevant suggestion locally where people are used to small gardens and the climate.

I live in a very different climate from you but have one suggestion, humbly offered.
Please consider adding colorful foliage. Interesting leaves will give you something to admire before and after the flowers have their season, and will add to the display while they are blooming.

Thanks edie,

I have actually decided that I am going ot put Cupressocyparis leylandii 'Castlewellan Gold' spiral topiary in the center of the four corner beds between the buxus and mass plant brightly colored (berry smoothie or peach flambe) heuchera as a ground cover between. That will bring color to the garden and will last year round without having to worry about trying to cram a gazillion different plants into a teeny tiny space for year round interest.

As it's such a refined area - would you consider something like Lonicera nitida in place of the Leyland cypress? Or Fuchsia standards? (Leylands have a well-earned reputation for being garden thugs...)

Well im certainly open to suggestions but I can only work with whats available to me here. (Netherlands) I certainly don't want something thats going to be a thug lol.

I agree with vetivert8 about the Leyland cypress. It wants to be a forest tree and you'd be pruning it constantly. There are lots of dwarf conifers which would be more appropriate. You're living in the centre of the universe regarding small garden appropriate shrubs. The Netherlands produces a large proportion of the world's new varieties and exports millions of plants to the UK and the rest of Europe. Your choice there is one of the best in the world. I'd get out to some nurseries and pick something more suitable after discussion a local expert. I wouldn't go for L nitida if you want a conical shape but if you aren't set on that Lemon Beauty is just what its name suggests.

Here is a link that might be useful: Dutch horticulture