How to fill holes in airing cupboard

[thiskitten]

We've just bought a house that's a got a really disgusting airing cupboard. Looks to me like under the hot water tank there is just a void (filled with so much dust it's filled my dyson and I'm not even finished yet!). And then around all the pipes there are just holes. How can I fix this?
We are planning on re-doing it all some point in the next few years, but for now I need to use the cupboard as it's pretty huge. All my linen is currently in suitcases in the Hall and they are getting in the way! I'm pretty terrified of spiders and this is looking like the perfect hideout for them.
I want to just fill the whole thing with expanding foam but my (potentially) better judgement is telling me that's a bad idea.

The important ones to fill are the ceiling ones as they will let warm damp air into the loft where it will condense and cause mould and rot.
I'd recommend expanding foam or some kind of sealant/caulk for the ceiling depending on the hole size.
The floor probably had the remains of the old water cylinder jacket which you have removed, you probably should board that up to prevent all your errant socks ending up lost there.