Hello I did my PRK laser here in Italy 8 years ago and I've done my 1 class there in Gatwick last month: no problem at all, got 1 class no restrictions.
But I was inside the limits, and the roule is to be inside the limits with or without laser intervention.
The limits, in your case, for astigmatism are 2 so should be better you ask to AME in gatwick, in case if you do laser, if you will able to repeat your visit. If they answer YES...NO PROBLEM!
You can also take an other way: you can go in USA, get FAA CPL flight licence and comeback in UK. On the basis of your FAA CPL they will do a visit but with limits more "confortable" and you will spend less money as well. So in that case -4 should be accepted.