Short answer: Yes hydraulic fracturing of the size companies would actually want to do is allowed. Long answer: In general what people are fighting against, unconventional oil & gas extraction (fracking), hasn’t been prohibited anywhere. In various parts of the world some potential aspects of these complex processes, such as massive slickwater hydraulic fracturing, have been temporarily limited in some way but this has usually been an attempt to placate extremely negative public perceptions in the short term while companies get on with exploring for these resources.
In the UK the Infrastructure Act 2015 is the only law which places much restriction on fracking, and only in the most trivial way. The main parts of the act relating to fracking are Section 41 which made “maximising economic recovery of UK petroleum” a legal requirement for the government and Section 43 which legalised drilling under land without the owner’s permission, a major stumbling block to the introduction of fracking in the UK.
However, Section 50 of the act does place some limitations on hydraulic fracturing, but only for certain depths and volumes. It limits the volume of frac stages (to 1,000 cubic metres) and the total size of the frac job (to 10,000 cubic metres), but only at depths less than 1000 metres (or 1,200 metres in a National Park, ANOB etc.). Given that the trend at present is towards lots of moderately sized frac stages, and the 1000 cubic metre limit is much less than the frac stages Cuadrilla plans to use in Lancashire (765 cubic metres), the frac stage volume prohibition is completely meaningless.
As can be seen from the graph on the left, which plots depth against volume of a frack job, none of the major shale plays in the US would be affected by such a law, since they are all below 1,200 metres in depth. Claims have been made that since parts of the Kimmeridge Clay shale in the Weald (Sussex/Surrey) are shallower than 1,000 metres that hydraulic fracturing in the Weald is prohibited. However, the the most pertinent comparison here is the Bakken shale play in North Dakota, a tight oil play to which the Kimmeridge Clay is often compared by the industry.
Typical frac job volumes in the Bakken are around 2 million gallons (7570 cubic metres), well below the 10,000 cubic metre (2.64 million US gallon) limit in the Infrastructure Act. Also significant areas of the Kimmeridge Clay which varies in depth across the Weald, from 600-1200 metres, is below the depth where there are any limitations, at least outside the South Downs National Park. The Kimmeridge Clay is also the shallowest of the shale layers in the Weald and the deeper Lias shale is well below both the 1,000 and 1,200 metre limit.
Most importantly note the tendency of frac job volumes to increase with depth, since the force needed to lever apart rock is very dependent on the amount of rock piled on to of it. Close examination of the graph reveals two trends one for oil (grey) and one for gas (yellow), with the tight/shale oil plays using systematically less fracturing fluid than the shale gas plays (the volumes for the Barnett and Haynesville Shale plays are likely skewed low because these older plays contain a significant number of vertical wells which require much less fluid to fracture).
Since the Kimmeridge Clay is much shallower than the Bakken, to which it is most often compared, it is most likely that the fluid volumes needed to fracture it would be significant less than the Bakken (particularly the micrite layers within it which are the target of more exploration at the moment). In the graph the Kimmeridge Clay (red) is show using about half the volume of the Bakken, 1 million US gallons rather than 2 million US gallons, which seems reasonable given that it is at about a third of the depth. Note however than the 2 million US gallon value is still well below the 2.64 million US gallon limit in the Infrastructure Act.
Finally, while the volume of each stage in a frac job can easily be manipulated to circumvent limits like this, by simply having more but smaller frac stages, a similar trick could also be played for the per well limit. Horizontal wells in the Bakken vary in length from 5,000 to 10,000 feet and so choosing a shorter lateral length for the well, and drilling more of them, could easily be used to reduce the frac fluid volume if it was rubbing up close to the limit. That said given how easily laws can be changed when the need arises, imagining that these regulations would be fixed in stone if fracking was allowed to take over the Weald would be somewhat naive.
In summary, while the Infrastructure Act 2015 has placed some limits on the depth and volume of hydraulic fracturing in the UK, these limits can be to have been specifically designed not to interfere with any actual hydraulic fracturing the industry might want to do. The act would not have prohibited any of the hydraulic fracturing which has taken place in the US or most . In the special case of the Kimmeridge Clay shale formation in the Weald, while parts of it are shallow enough to be covered, the generous limit of 10,000 cubic metres (2.64 million US gallons) per well means that any hydraulic fracturing which the industry would actually want to do would also be allowed.