Will I get blacklisted by agencies if I back out of an offer

[Not So Wise]

There are no such things as blacklists except the one a particular agent (not the agency as a whole) might keep.

Basically because they are unworkable, five minutes after one was created some clever sod would load up all the best (highest paid) contractors he knows to screw all other agents out of getting a possible commission by forcing contractors to go through him

And until you have signed the contract not much agency can do if you change your mind (and same applies in reverse if they decide not to give you the contract) and even after you sign very rare for them to do something.

Mainly because unless contract is worth millions (and your Ltd is also) the cost/benefit ratio of suing just does not make it worthwhile

[Wanderer]

I am tempted to do as you say & sod the agent, but unfortunately they are one of the large preferred suppliers into a niche market (high security clearance government).

You've upped the ante a bit now.

Generally, there isn't much goodwill between agents and contractors but if you're unlucky it may prejudice them against you for being a bit of a time waster. Rest assured, if the client wants you, the agency will get you. But if it's 50/50 then it might tip the balance against you. You've just got to do what you think is best. If you do blow them out then make sure you come up with a very plausible cover story.