In the UK, with what I consider a restrictive speed limit on commercial ebikes, you might want to consider putting a motor on an existing bike. There are a number of shops that will do that for you, and most will build one that will keep up with your road bike. If you start with a good light weight bike, and go with a smaller battery, you're looking at 10-12 pounds of added weight, somewhat better than the overbuilt 50-55 pound e-bikes commonly sold.
I'm not talking about monster kits either. Just the same 250W motor, which is legal, and most of them can do 18-20 mph unfettered on 36V. Hardly earthshaking speed.