Don’t be mislead, when you are looking for vintage telephones in the UK 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 usually 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 (up to 95% or the potential value). So the important thing here is not to just rely on the photographs of the vintage telephones but to ask the seller to confirm that there are no hairline cracks. If they are a reputable antique/vintage telephone restorers they will be happy to confirm this. If you buy from an antique centre always make sure you check the Bakelite in a good light, ideally look at the telephone in 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 those on online auctions (or generic antique dealers). If you can’t confirm that this service and calibration has been done correctly your best option is to offer less for the vintage telephone (about 60% less) and then have the telephone serviced elsewhere.
Always do your homework and research, there are some dealers that sell via places like eBay that you really would not want to service your “pride and joy” telephone and that really don’t know what they are doing with these iconic 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 to get a good vintage telephone restorer.