From my own experience, doing the PPL as quickly as possible pays off. You will take far longer to do it if you're only taking lessons once a week or so. The effect of the unpredictable weather in the UK (especially as we go into the winter months) can compound this problem. The cheapest way of doing it may well be to head out to Florida/California and do the whole course in three or four weeks.
That said if you're wanting to go on and take the CPL in the UK, I've spoken to quite a few commercial students who have done their PPL and hour building in the states, and then struggle with the CPL in the UK due to the differences in teaching. They end up having remedial training in the UK which can quickly erode the cost advantage of doing it in the states in the first place. On that basis, if you do your PPL in the states, you might want to do at least some of your hour building post PPL in the UK to get used to the airspace and environment here before starting the CPL training.