I suppose it depends on whether you are keen to get the best value car (in terms of price and also insurance) or are prepared to spend more on a particular type of car which might be considered more stylish or desirable and which might have higher insurance.
The Aygo/C1/Peugeot 108 (all the same basic car) are likely to be the cheapest cars to buy and there are certainly models within Group 1 which gives cheapest insurance.
Slightly higher priced, and more robust in my view are the VW UP/Skoda Citigo/Seat Mii (all the same basic car) which also have Group 1 insurance models with small engines which are very suitable for new drivers.
You can go for a Mini or a Fiat 500 or similar ‘lifestyle’ car and lots of teenagers are drawn to these. They often cost more and some of the models are in surprisingly high insurance groups.
Polos, Fabias, Corsas, Fiestas - all associated with younger drivers, but often a bit bigger engine and often a couple of groups of insurance higher up.
Each family needs to decide what their budget is and what the priorities are. Some will be willing to soend several £k more in buying a car and also more in that first year of insurance too. Black box type insurance can be had for the Group 1 cars mentioned for well under £1k if a parent or two are added in on as named drivers, especially for a girl and especially if they have either turned 18 or have held their licence for even a few weeks or insurance is being taken out for 2+ weeks time rather than tomorrow. But equally you can pay £2-3k or more for insurance if you go for a higher insurance group car.
Most people are looking for the best value for car insurance for their new driving teens. But often they don’t buy the cars that can deliver this.