The zoo is apparently “too big” which can’t bode well for pretty much every major zoo in the UK. As Twycross is definitely in the middle of the market.
There is an argument that the bigger zoos should generate enough revenue for a decent "rainy day fund" (similar to the government not covering anyone's wages above £2,500 per month). A few years back it used to be Twycross' policy to aim to have enough cash reserves to close for a sustained period (it may have been up to a year, possibly following the foot and mouth closures)-I'm not sure whether they ever achieved this.
The reality, of course, is that most zoos roll spare cash into new enclosures or facilities. I suspect this policy will change in the future.