Let's first see if the UK CAA licence will be treated as an equivalent of EASA for insurance purposes. If yes, it will hardly make any difference to an ATO which one to offer. If not though, those intending to fly commercially will likely opt for EASA as it will give them greater flexibility in future employment. Therefore, ATOs geared towards commercial training will likely be predominantly EASA, while those who mostly do PPLs and serve private pilots will stay under the UK CAA. That's my semi-educated guess on what will determine the general direction in which an ATO will choose to go. What will any one particular school choose though is in the hands of its management. Therefore, it's best to ask them directly.