If you've done it a long time ago it's all changed anyway
Somewhat strangely, you check in at the domestic terminal who obviously arrange for your luggage to go straight through to the UK airport. Once again you will be greeted as you enter the terminal at Istanbul by a THY employee who will be calling out for any Dalaman arriving passengers who are transit to UK. You are sent through in the correct direction and clear passport/visa control and from there it's laugh at the prices time till your gate is called.
One of discoveries of joy on the round trip this way is that if you leave your car in the long term car park you are very handily positioned for entering the domestic terminal to start your epic journey. Knowing you and Pete you will be taking fairly empty suitcases here, and rather fuller ones back. What a pain in the neck it was for me to arrive back at Dalaman, ham and cheesed to the hilt, to appear from the international terminal and drag my case and bags all the way back to the domestic terminal, where the car was!
So get a transfer or someone to take you and collect you