Welcome to the forums.
You have certainly had the best four stroke 125 - If you don't want the expense of a 2 stroke, buy another one, its only real competitor is the KTM as far as looks go and speed go.
The GPR 125 was born out of Derbi's 125GP efforts, so handles well, but the new ones are 4 stroke, and much slower - early bikes are 2 stroke.
The Mito 125 is one of the fastest 125's made - Think of it as a baby 916, is how most people describe them, and it deserves that reputation. They start at 1000 pounds plus, but that is my end of the market. The new one is called the SP525, and can easily be mistaken for a Ducati 998. (I've done it on numerous occasions.)
The Varadero is a big 125, and according to bike magazine, if it was 30 mph faster, it would be better than the 1000cc version. Doesn't look as nice, but thats my opinion.
Is the budget just for bike, or does it include replacement gear, insurance etc?
Way under your budget, but way over your mileage, I have a '53 RS 125 for sale. done 25000 miles, and the ad is on the buying and selling bikes section of the forum. For 800 quid going by what the bills should work out at - I may wind up needing more, and I would love the chance to buy it back, so you won't make a loss if you decide to move on.
I hate central heating...
You will quickly find that the quickest way to get a response is to post in General anyway.