your certainly allowed on the course with it, and most schools also will give you a discount if you have one. OAT takes 15hrs off and FTE is 20 I think.
I'm not sure if there is any point in doing it. Getting the PPL will cost quite abit of money and if your planning to go integrated then you may as well save it. I have just got my PPL since I was planning to go modular when I started however I am now thinking of doing integrated (ah the decisions!
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Gliding is great tho and will teach you alot about proper flying, and the weather! Make sure you come to the inter-uni task weeks too, since there great fun and you get to fly around a different bit of the country. This years is at sutton bank I believe.