I notice that this is a very old question, and hopefully the person who originally asked it now has a PhD! However, as it may be useful for others, the answer is yes in certain research areas, and no in others. It can often depend on where you are getting your funding from. The AHRC and ESRC usually require that you have an appropriate Master's degree to be eligible for their funding. I would concentrate on finding the University and research area you want first, and then worry about that later.