070xx started off as a a redirecting number, especially where one used quite a few mobile numbers, but that has lost its way.
Whilst they can be used genuinely by WGs, this is now rare, as it is now far easier and cheaper for them to replace an exisiting PAYG number with another easily-bought PAYG SIM Card.
Most of the time, it tends to be same dodgy escorting sites who end up advertising 070xx numbers, as they may be getting a kick back when these numbers are dialled.
As you mention, dialling 070xx does not use up any part of your bundle, so you will be charged a high pence per minute charge, which can be avoided.