Hi Vinny,
I use the Oxford ones same as you and have also found they blocked access to the rear pegs if mounted in the usual way, but we found we were able to mount them further back to allow comfortable access.
This has worked fine for several tours including our recent honeymoon ride from Santander in Spain back to the UK.
Mounted this way only two of the three straps goes under the seat, whilst the third runs over the fairing at the rear of the bike (between the seat and the rear grab rail).
Don't worry, they're not going anywhere, I've done thousands of miles with them like this and so long as there's a good couple of straps under the seat plus ancillary bungees they're fine.
The main issue with this set-up is that when heavy they rest on the indicators, but you can use bungees to support them at the bottom and take the load, keeping the weight off the indicators.
I tend to tie them to the rear foot peg hangars at the front.
To support the rear I use bungees attached to the base of the pannier on the inside next to the bike, then run them under the pannier and up and around it's outer side, then through the carry handle and then under the pillion grab handle and then finally attached to the net which I have over the top of my top box.
I'd be interested to hear how other folks solve this problem as although it works anything which increases pillion comfort is welcome!
Hope this helps, Andy