If you won't go unvented then it looks like the best option is to damage some floor and fit a leccy shower.
Replacing Your Elson with a tank and cylinder and shower pump could work out more expensive than replacing a bit of flooring.
Get someone in to have a look at the job ... They might figure out a better route for the wire.
Remember you can't make an omelette without breaking eggs.
Other idea is fit a combi which is less reliable, even more expensive and nowhere near as good as unvented which is by far the best bet from the information you have given.
I've got a combi boiler which gives pretty poor hot water pressure, but I've installed a Wickes Hydro shower which gives me power-shower performance with no pump. It's fantastic and may work with your setup. It uses the Venturi effect to 'suck' the hot water through using the cold water pressure.
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