First walk the streets around and make sure you do occasionally see some heather in neighbours gardens.

I live not far from you on a similar soil and grow heathers no problem. But my soil has a sandy loam topsoil over the clay with stones, and so might be better drained than yours. Despite their reputation for growing around peat bogs, heathers do actually demand a well-drained soil. What can happen is that if you have a poorly drained soil, dig a hole in it, fill it with a suitable planting mix, is that these planting holes can become underground puddles. In such conditions, the trick to growing heathers might be to grow them in raised beds. (Btw, include some sharp sand with the ericaceous compost in your planting mix. Sand is acid.)

The other mistake you might be making is to be buying non-hardy heathers. The prettiest looking heathers for sale are often South African heathers. These don't normally survive the winter outdoors in a Hertfordshire garden. The trick is to make sure you are buying a variety of a native heather, or one of certain known hardy imports.

The two really common native species of heather are Calluna vulgaris and Erica cinerea. There are lots of varieties for the garden. If you buy varieties of these, they should do well (subject to the soil conditions issues and drainage issue above). Some are more insistent of very acid conditions than others. There's a common white flowered one (I forget which species) which is particularly tolerant of not-very-acid conditions. It even grows in chalky conditions.

Erica ciliaris is another native. You should alos be able to grow Erica lusitanica, Erica arborea, Erica carnea.