I can't comment on Total Immersion, but I know there are a few around here who have used there trainignand I'm sure they'll be able to advise you on that!
I live in Derbyshire as well, near Chesterfield! I try to use my commute to work as part of my training, which means being up at stupid o'clock and leaving the house in the dark! Would this be an option for you! If you're pressed for time it's a great way of getting the miles in! You could maybe cycling to work & get a lift home then vice versa!!
Fortunately I can use the 5 pits trail for a large part of my bike commute which keeps me off the main road. I use my MTB lights when I have to cycle in the dark! They work a treat for what I need. Especially when there is a lack of street lighting!
Speaking of which, time for me to hit the turbo!! I know I'm sorry I was remiss in not being out at sparrowfart this moring in this glorious spring weather!! But since I raced my MTB yesterday and having an aching knee this morning, I think I'm allowed a gentler day! You know I really wish I had a little red bike with 50 mm wheels and 11 speed cassette to make it easier!!
Oh wait, Ironman isn't meant to be easy is it!!! Otherwise it would be called Football!!!