Wow. Growing Spinifex in the UK is quite ambitious! It's quite an extremophile, certainly hates the UK climate and soil types. It's also quite a large plant, although I suppose you could harvest it when it's small. It's also an absolute bastard of a plant in terms of providing you with an ample supply of regret when you're silly enough to touch it.
Mulga is tougher, it grows in reasonably similar conditions to Spinifex (extreme temperatures and moisture levels and nasty sandy soil depleted of nutrients) but will better cope with UK conditions and you might manage to get a tree growing there as long as you do things in a clever way (plant it against a south-facing wall etc). I was surprised to see one growing at my university campus in Melbourne, Australia, which has a similar climate to the UK. You wouldn't get Spinifex to grow in any normal place in Melbourne
Even in Australia few people bother using these plants in enclosures and if they do it's just for novelty. Unless you have a living Mulga tree it's quite pointless using Mulga - it looks sort of distinct as a living tree, but isn't especially pretty or anything, and as cut wood it's just basic wood, but, hey, if you're keen just for the fun of it, go for it!
You'll probably have more luck finding these seeds by talking to plant enthusiasts rather than reptile enthusiasts. Because Mulga and Spinifex grow in extreme conditions, the vast majority of Australians live nowhere near it, and the very few herpers who are into it (I've never heard of anyone bothering to use Mulga, a few people are keen to use Spinifex though only a few of that few keep bothering after trying it) just go collect it rather than grow it. Even in Australia, herpers sometimes ask me to collect Spinifex for them because I travel around and get to where it exists, while most Australians have never even seen it
Mulga was one of the food plants I used for my animals during my honours project at university, it didn't do well in the greenhouse and I had to use alternatives. Some of the other Acacia species from the same regions did much better. Spinifex... ugh, I wouldn't even try, but if you do, respect to you and I'd love to see how it goes!
