This article doesn't provide enough information to be useful. Is "thousands" a lot? It depends on what kind of drones we're talking about. Ukraine produces on the order of ten thousand military drones per day, as does Russia. So the UK sending "thousands" one time might be insignificant. On the other hand, thousands of properly equipped Reapers would be enough to allow Ukraine to defeat and possibly conquer Russia—but nobody has or will ever have thousands of Reapers, which would cost on the order of 3% of the GDP of the UK.

So the description in the article is so ambiguous that it covers the full range from "insignificantly small" to "implausibly large".

https://news.ycombinator.com/item?id=45223912 ballparks the program at US$10M.