Although still an 02 customer pay as u go do u still get true 4g speeds even on pay as u go. I've recently just got a 4g phone and have done a few speed tests averaging about 15 meg. To get the proper speed will I need to go pay monthly or 30 day sim

No, that's about average for o2 4G speeds, it doesn't have the network spectrum like EE or Vodafone so very high speeds aren't going to happen.

To add to that, you might be lucky and get higher speeds than this but 10Mhz of 800 can't go faster than 75Mbps and with overheads, signal fluctuations/quality your top end is going to be 65 with the wind behind you (just you on that particular cell doing something). 15 - 30 probably more normal.
EDIT: Oh and I believe they are doing aggregated 1800/800 (did someone say that at some point?).
So 5Mhz 1800 you can add another 15 -30 to the total.
Assuming only you on the cell, ratchet strapped 30 metres up to the antenna panel, on a mast with fast backhaul, then you might see 80 - 90Mbps, once every solar eclipse.