Lidl has rolled out its own "smart" bike helmet. It is competitively priced at £39.99 (as of April 2019) and features a rear light and indicator lights. The rear light can be either continuous or flashing, and all light controls are accessible from a remote control that fits onto the handlebar. From the picture on the box, the remote control doesn't appear to be terribly weather-proof, and I'd be surprised it it lasted more than six months of regular riding in UK weather!
The helmet itself is quite a basic affair and comes in just two sizes (S/M or M/L) with a ratchet mechanism at the back to get a snug fit. It is fairly light considering the extra electronics and battery that are packed into it. Actually, the battery is the elephant in the room here: we don't know what form it takes or how to recharge it. Definitely something to investigate before purchasing.