I have a conventional boiler & vented HW system & I’m not that familiar with combi systems; I know they operate at mains pressure so, in theory, the hot water pressure should be sufficient directly from the boiler & again, in theory, all you need do is match this with a separate mains cold feed to the shower mixer. The question is how high up your new en-suite will be, how this will affect the water pressure/flow rate & how this will ultimately govern your choice & performance of shower fitting. The other problem with combi boilers, particularly older ones, is that they can’t deliver enough hot water to keep the shower running satisfactorily if someone turns on a hot water tap elsewhere! A HW storage tank is one solution but I’m unsure if these must be unvented (DIY not allowed) or vented; either way tank, pipe work & fittings is going to add considerably to the cost; hopefully, someone else with more experience on combi systems can chip in or correct me if I’m wrong! Have you though about installing an electric shower?
I was in the situation where my whole installation (including the CH heating system but not the oil boiler) was old & completely shot so I ripped it all out & installed a 2nd 50 gall hot water (I have a solar heating system feeding the original), 1 x 50 gall cold, new 22mm pipe work from the tanks to the bathroom & en-suite, full flow isolation valves, thermostatic control valve & a 3 bar, whole house pump; that little lot alone came to well over a grand & I installed it all myself! But it delivers water to the 2 power showers & bath we have at one heck of a rate & the showers work great!
You’re confusing Planning Permission with Building Control, they both live at your main local council office but the 2 are completely different; you don’t need PP for what your doing but you will need to comply with the relevant Building Regs. Re BC guidance; they can be very helpful but their role is not one of a free Design Consultant. How much they are prepared to help & advise you will depend on how cooperative your local guys are. Ideally, you need to employ someone who knows & understands the regs. to produce a specification your trades can quote/work to or do what everyone else has to, download them (free), learn them, plan & supervise the work yourself. You need to submit a building notice outlining the work your doing, a realistic cost & a fee based on that cost; a BCO will then visit to inspect (mainly drains, electrics & insulation) as you complete various stages & either pass or otherwise comment on what you’ve done. If the en suite is in an eaves top floor, do you still have a loft space? Has the loft been converted in the past or are you doing that now? Either way, if you’re putting a new room & a Velux up there, BC may require the eaves insulation to be upgraded to current standards! Velux units themselves are not that expensive & are fairly easy to install but, again, it will have to comply with BR’s & a DIY installation will also need inspection as far as I’m aware.