I inadvertently started a small brushfire in our wood during the very dry spell last year. My chopped bramble burning spread from the firepit to the leaf layer, and the standing brambles started to flare too. Fortunately it was slow, and petered out, but I had a moment of panic. It seems large deep continuous drifts of understorey plants such as bracken or bramble can afford a way for flame to spread in unusually dry periods. The answer would seem to be to break up these areas with strimming / brushcutting to create paths and firebreaks. If you have tinder dry conifers, its going to be a worry, but wide rides will both lessen the risk and introduce more light for flora and wildlife.
BTW I no longer burn brambles, I find using a mulching blade on my brushcutter reduces them to small fragments which rot down to fine compost in a short time, it cuts out both the necessity to handle them and having to dispose of a mountain of prickly debris.