Hello Carl,
I think ADSL over ISDN had been developed for Germany for the wide deployment of ISDN (they were flat rate on ISDN)
I suppose in UK you should be OK with ADSL over POTS (analog phone line)
the difference is in the way bandwidth is used on copper: with ADSL over ISDN some lower bandwidth subcarriers (tones in DMT modulation) are not used to leave space to ISDN BRI and to be compatible on the same wire pair.
You can give a look at DSL forum to get more information
Hope to help
Giuseppe