Hibiscus does not grow well in wet, waterlogged conditions, preferring soils that are moist but well-drained. Loam and sandy loam soils tend to be the best. If you have too much sand in your soil, you can improve its texture by incorporating mulch or other organic matter. A pH below 7.0 indicates acidic soil, while a pH above 7.0 means your soil is alkaline. Hibiscus prefers neutral to slightly acidic soil. If necessary, you can lower the pH of your soil by mixing in sulfur or a sulfur compound like aluminum sulfate. Lime is commonly used to raise pH, and common liming materials include dolomite and ground agricultural limestone.

Have a look at this first.
http://www.aces.edu/dept/extcomm/specialty/feb28kgard.html

I really wouldn't try to repot it when its in full bud or the buds may fall, if there are any left.. Stand the pot on a saucer or inside a decorative pot but with the base of the pot raised so there is a gap underneath it. The woodlice are unlikely to come out of the pot that way and you will avoid the pot accidentally being left standing in a pool of water.

When you do repot it after the flowers have finished ordinary standard potting compost from a garden centre will be fine and is most likely what its in already - there will be no need to worry about pH if you do that as it will be suitable.
All the woodlice will do is eat mostly dead roots etc and they are completely harmless otherwise. If you really cannot stand this idea you could keep your fingers crossed that it is slightly pot bound and gently invert the pot, supporting the plant as you do so. If the rootball shows signs of falling apart then immediately put it back. But if it seems to hold together let it slide all the way out, supporting it with the other hand all the time. You should be able to knock off any woodlice you see. You can very gently loosen the base to encourage a few more out.
(You'll perhaps feel happier doing this outdoors...)
The put a little new potting compost in the base of your pot - how much depends on how much has fallen out - and settle the plant back into it.

Gently does it and you might be lucky if you're careful. But it would be safer to wait until the flowers have finished if you can bear to, especially as its still recovering from the sudden atmospheric change from outdoors to inside..

Instructions contained in the first answer to your question are largely irrelevant, since you're not growing your plant in the ground, but I'm curious as to why you want to bring it indoors anyway - if its Hibiscus syriacus, that's hardy out doors and should have finished flowering by now, so which variety is it?

In the States, the appropriate potting compost is sold as "Cactus Mix", but I am not sure what it would be sold as in the UK. A good simulation could be mixed up from 80% good indoor potting compost, plus 10% each grit and pumice (or volcanic cinders). Hmm...I also don't know how available the last two ingredients are in the UK. Perlite would also work, but isn't ideal.

You can get cactus compost here in the UK but its very dry and sandy. I've never seen any indoor potting compost - how does it differ from the ordinary stuff?

Usually slightly coarser--i.e., less silt or ultra-fine sand--and the better kinds are normally free of fungus gnats, though that's dependent partly on how it was stored and handled by the various hands it has passed through.

You can buy Houseplant compost, Steragram - its expensive though. I bought it once, in desperation, when they first started changing multi purpose compost and it was all full of rubbish and bits of wood. Can't say it was worth the money - Wickes own multi purpose is much cheaper and just as good.

Dry and sandy is just fine for Hibiscus rosa-sinensis hybrids, which is what I think Flowertree's is. I wish that I could get a dry and sandy compost for cactus, here. The brands available to me have much too much peat moss and/or coir in them--good for sedums and tropical epiphytes, but not so much for things like Astrophytum or Melocactus.

I used to make my own in the days before you could buy it. as far as i remember it was John Innes 1 with added coarse sand and some crushed flower pot.