I don't know about the UK but here in the States several of the big name builders will lease props. You can likewise check with those that produce theme park shows as does Paul Osborne and Mark Wilson, they frequently have duplicates that are available for short or long term lease.
Understand, the going rate on a prop is typically 10 to 25% of it's current retail/replacement cost and too, there is usually a stiff up-front deposit especially if you are basically "unknown" to the industry and young. In fact, character references from noted pros willing to vouch for you would be a very good thing to have in hand.
To be honest, your best bet would be to buy used via sites like magicauction.com or else build the props yourself. DO NOT BUY the Chinese or South American knock-offs in that they are unsafe and usually won't endure more than a season or two of road work, if that. Buy legal, professionally made equipment. It might cost a bit more up-front but you'll really be glad you did it.
If you do choose to build you want to get as many books on building props as you can, starting with the Paul Osborne books. If you eMail Paul about a specific effect, he may have the plans available, which typically sell for about $25.00.
QUESTION: Have you weighed the headaches tied to big props yet?
Ignoring up-front costs you have costumes that must be dealt with and in some cases the need for a specific type of set/back-drop.
Which version of the effect? is you next big issue. . . there is a huge difference between a Bill Smith made Sawing Cabinet vs. one based on Abbott or even the old Thayer work-shop plans. The difference centers of deceptiveness. . . a Smith or Gaughan table-top barely looks 2" thick and is flat while the Abbott base is pushing 3" and looks it. Then you have the Owen design which is a tapper or "wedge" base vs. the flat design or John Gaughan's famed "Diamond" design popularized by Mark Wilson (which to super sweet).
The cases needed for a prop will run you another grand or two, depending on the size of the item. There's storage and of course, supporting a steady full-time and committed crew. . . this is a biggie in that rehearsal time gets costly and having to train a new team with each gig is a great way to get nowhere but broke really fast.
I know I've touched on a few things outside the parameters of your question but my goal is to help you with your decisions and maybe encourage you to take you time to build-up to the level of being able to effectively include the bigger pieces. You can't do big illusions just anywhere and certain effects come with some very specific complications.
I've been working with a lad here in the States and shown him how to build a low cost but very solid illusion show. It's not his dream show but it is the style of program he wants to work towards. It might very well be a direction you'd benefit from in the long run in that you can put your cash where it will be doing you the most good vs. throwing it at a dream that may be impractical for now.
Certainly, if you have a special situation where a mega stunt would prove reasonable because of media hype, etc. then go for it! But weigh the venture against the gains and consider what is most practical right now.
