If your HWS tank is a standard 40-ish gallon tank, there should be room for the 27" heater, which will give you a greater depth of hot water for your length of showering - especially if another person wants to shower soon after you.
Plumbers may correct me, but the 11" heater is there to heat the top section of water in the tank to back up the boiler heating coil.
I assume that at present you have a blanking plate where the immersion heater is to go?
Some months ago I had to replace my immersion heater and I learned a whole new lexicon of profanities!
After trying MANY different techniques, I ended up using the biggest, best-fitting spanner and long lever, with sustained but not massive pressure on the lever, occasionally giving it sharp taps to jar it undone.
PLEASE have the tank full of water before you start to undo it, but only to loosen it! Once loosened, drain the tank somewhat to lower the water level and remove the cap, to be replaced by your immersion heater, obviously. The body of water strengthens and supports the tank whilst you apply the Desperate Dan measures on the cap.
Do not be tempted to use sealant on the threads of the heater - that is why mine was such a son-of-a-bachelor to get undone!
Good luck.
Ron.