Please could you suggest a shrub

Camellia should be fine for some time, Fatsia japonica is another option. If the plant you choose is in a relatively small pot, best pot it into something in between the size of your iron pot and the pot it was in originally until it grows on a bit, rather than just putting it straight into a very large container. If its against a wall, Fatshedera lizei, provided you can give it some support.

Camellia for spring flowers.
Hydrangea for summer - white ones look great against cast iron.
Clethra alnifolia also for summer flowering but some have nice leaf colour in autumn.
Viburnum for winter flowering.

Try to think about what will provide interest when the shrub isn't in flower - do you want bedding plants/bulbs or does the shrub have something about it all year round.

A male skimmia such as Black Knight would be OK in a large pot for quite a long time. Its flower buds are red all winter and in spring they open into sprays of small white highly scented flowers. Before it fills the pot you can underplant with snowdrops and annual begonias so you have most of the year covered as far as interest goes. The stems with buds are good in flower arrangements too.