I did a PPL some years before starting the integrated course at Oxford. I'd say it helped quite a bit. However the cost of it will add to what is already a very expensive course. So I would recommend just doing enough lessons at a flying club to go solo. That way you can see if you really like it.
Unbelievably there are people who start integrated training without ever having been in a light aircraft. I know more than one person who was chucked off the course. Everyone I know who has failed the course failed in the very early pre-solo stage. So if you can get that squared away before you commit to the ATPL, I'd really recommend it.