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Hornets are pretty quick, I agree with soul that you will learn more (and potentially live longer) starting off on a smaller bike. A 250 or 300 will be more than capable of meeting and exceeding many speed limit you find in the U.K.
For the other half have you considered a more modern Vespa/piaggio thing? Old Vespas are fun and nippy but things like reliability, gear change grip shift and clutch on left hand, throttle on right and pedal brake could be a challenge for a noob.
This is absolutely what I would do otherwise, I just thought it would be kinda nice to get the same, albeit smaller version, of my dad's bike.
But I suppose it comes down to what's on the market locally when I'm looking too