I have just bought a new lg 4K uhd tv but have an old panasonic DVD home cinema/ surround sound that only has scart output. I've bought a scart to hdmi converter which will arrive soon, and I hope it will produce pictures whilst allowing sound from surrounding speakers, but will I be able to use the surround sound speakers for regular tv watching?
If the TV had a Scart connection it would have audio out from the TV.
It sounds like the converter you purchased was to connect the Panasonic for playback on the HDMI input of the new TV. That won't allow you to get audio into the Panasonic from the TV.
Most likely you will need a digital to analog converter like this one http/www.parts-express.com/parts-express-dac-digital-to-analog-converter-(51-compatible)--180-999
to connect the optical out of the TV to the aux input of the Panasonic.