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Do the whole thing.
There’s no joy in a 125, you get blown around above 40mph (esp in the fens), they’re not quick enough to feel safe on 60mph roads, you feel really vulnerable as you’re at the top of what they’ll do. It’s enough to put you off riding.
A big bike (100bhp is perfect) makes you feel much more comfortable as you can very easily keep up with traffic without pushing its limits and nip away when you sense a driver isn’t being careful. On dual carriageways you need to be able to add 10mph instantly and move forward into a space to overtake. 125s on dual carriageways are horrid.
ETA: Do it!
As in you’ve not yet done your CBT? If so don’t worry that you feel terrified and bored on a 125. It all makes sense once they put you on a 600.