You will see a wide range of prices for British Bakelite telephones. The main two things that impact the prices are the condition of the Bakelite and the mechanical service and restoration that the telephone has received.
The condition of the Bakelite is relatively easy to spot, its a lot like pottery where hairline cracks or significant chips above a couple of mm (nibbles) will massively impact the value. Don’t just rely on the photographs ask the seller to confirm that there are no hairline cracks, if they are reputable they will be happy to confirm this. If you buy from an antiques centre make sure you check the Bakelite in a good light (ideally sunlight).
It is much harder to determine the quality of the mechanical service and internal restoration that the telephone has received. This is one of the main reasons why you will see a big difference in prices from reputable dealers and online auctions.
Always do you homework, there are some dealers that sell via places like ebay that you really would not want servicing your pride and joy and that really don’t know what they are doing with these telephones, even though they have been in the “Antiques” business for many years. Research, research, and more research. As always word of mouth is always the best recommendation.