You don't need to use any of your allowance to use Vodafone's own speed test app (which, IME usually reports a slower speed than using Ookla's app!) but you do need to have at least some data.
And, I just logged into my PAYG account and the following top-ups are available. As I thought, there are still some non-4G plans (but not many - last time I am sure there were duplicates for the 1GB and above plans that didn't have 4G).
All the following plans are considered 'iPad plans'. Perhaps there are different plans for mobile broadband, even though I can't see how there's any actual difference.
£3 - 200MB/24 hours/no 4G
£5 - 250MB/30 days/4G
£10 1GB/30 days/4G
£20 3GB/30 days/4G
£8 100MB Europe/24 hours/no 4G
£25 100MB Rest of World/24 hours/No 4G
£15 2GB/30 days/4G
£25 6GB/30 days/4G
So, you could pay £5 and get 4G access, then go to town on speed tests using the Net Perform app.
It is IMO silly that until you add a 4G data add-on, you're not allowed to connect to 4G. Obviously that's how they stop you using 4G - but it must result in people thinking 'no point paying for 4G, as there's no 4G around here'.