A lot of online stores in the UK are sold out, although my local Scalextric stockist (no website) had a box of them on the shelves the other week. There are some available at Jadlams for £3 more than RRP, a 20% price hike. They are still available in the US Scalextric store and with Scalextric retailers in North America. It is definitely worth visiting, phoning or emailing your local Scalextric stockist, especially if they don't have an online store. The list is here: https://www.scalextric.com/uk-en/stockists/
The EasyFit Digital Plug is listed on the UK Scalextric website as 'Expected March 2018' - that's next month.
I don't know about the C8515 being Scalextric's most popular product. I know excess stock has been heavily discounted in the past, so that suggests it hasn't always sold well. The digital plug is now being produced in a new version (rev H). The new version appeared last autumn in relatively small numbers and was snapped up by enthusiasts keen to try out the upgrade. Then ARC Pro came out in November and sold very well, encouraging plenty of people to try digital and buy chips to upgrade their analogue cars. That combination of modest initial stock levels and bigger than expected demand from old and new digital racers has probably been the reason for the scarcity of the chips in the UK, especially online.
The way I see it, Hornby's priority is the sustainable development of all their popular brands in what is a precarious market. There is plenty in the financial and trade press about their efforts to turn things round. If that means smaller production runs and a more organised warehouse less full of unsold stock, then I think we're going to have to accept that stuff like the chips and the ARC Pro controllers might go out of stock from time to time. Both of those shortages are symptoms of the success of ARC Pro before Christmas and that has to be really good news for Scalextric and everyone who loves our hobby