A 5yr plan to invest £100k in an ISA…brilliant! It is the the dream objective to max out that £20k annual limit consistently.
Just remember that would if it were all in on your stocks & shares ISA, you couldn’t add more to a cash ISA etc. (why you would want to is another matter altogether) 
If you can achieve those savings, I would suggest you will see a pretty nice return over time. Let compound interest do its thing & with some wise picks, you’ll be laughing.
Good luck to you🤞