If you do your bike test on a 125cc bike, you will be restricted to 25 kW (33 bhp) for the first two years, even though you are 25. This need not neccessarily be a 125cc but can be a larger bike, provided that it complies with the 25 kW limit. Many medium sized bikes can be fitted with an approved restrictor kit too so you'll have a good range of options. For info, a 125cc learner machine is limited to a maximum power of 11 kW.
As you are over 21, you do have the choice to take your test on a larger motorcycle with a power output of at least 35kW, which would then enable you to ride a bike of any size with no restriction.
A couple of tips whichever route you choose:
1. You must take both parts of the pratcical bike test on the same category of bike - i.e. you can't take Module 1 (off road test) on a 125 and then Module 2 (on road test) on a larger bike.
2. You can complete your CBT on a smaller bike and then go on to take both parts of the practical test on a larger bike.
3. Once you've passed your CBT, you can legally ride a bike upto 125cc unaccompanied on the road with L plates but are not allowed to ride a bigger bike on the road unless you are accompanied by a DSA DAS approved instructor.
Regardless of which route you take, I would suggest that getting proper training is essential.
I think your approach to take your test on a 125cc and then use the two years to build up your skills and experience is extremely sensible.
Once you've done your normal bike test, I would highly recommend continuing your learning with some advanced training. I took my first advanced bike test with the IAM on a Honda 250 at the age of 19 so it can be done!
Good luck with training for and taking your test. Keep us posted on your progress and shout up with any questions you may have.