OK, I figured it out. At http://tutorials.zen-cart.com/index.php?article=28, it says
In the UK, the Payflow Pro service is actually bundled as a hybrid service with Website Payments Pro, connecting all the transactions to your UK PayPal account, and all monies are deposited to your PayPal account, instead of directly to your bank account.
So yes, WPP in the UK requires (or in some sense is) payflow.
I generated some PayFlow credentials by going to "Request API Credentials" in my business account on the paypal site, going for option 2 - "Payflow Pro" and "Set up Payflow Pro API Access". This gave me values for the 4 fields, which I entered into the settings under the paypal express checkout module after setting the "Paypal Mode" field to "Payflow-UK". It actually didn't work at first (giving the same error as before), but one of the following two things sorted it out:
- Generating a second set of payflow credentials and using those instead of the first ones
- Going to the Website Payments Pro module in zen cart, and clicking edit and save
Or perhaps I'd simply mis-copied the first lot of credentials and got it right the second time. Just listing those on the off-chance it might help someone else in the same situation. Anyway, sorted!