The Deviation classification only stays on your medical certificate until you finish commercial flying training - once you have the CPL/IR or fATPL it is lifted i.e. your medical certificate becomes a bog-standard Class I. The only restriction is that your training must take place on UK-registered aircraft - not an issue if you're doing your training in the UK anyway. I f you wished to do training elsewhere, you would have to get the permission of the appropriate authority in that country, as I understand it.
Case study: I fall just outside of the astigmatism limits for a Class I but within the RENEWAL limits. I currently have a Class II and have recently started my PPL training. Upon completion of the PPL I will be able to be examined to renewal standards for the Class I. Assuming I stilll meet the standards, I'll be issued with a Class I (Deviation) - this allows me to do commercial training. Upon completion of the commercial training the Deviation will dissappear.
Lots of hassle and loads of hoops to jump through but, in this case, you have work with the system I suppose.
Good luck,
V1R