Checking whether my broker's swap matches the rate differential

[FXS London Scalper]

I got curious last month and compared my broker's overnight swap on GBPJPY with the interest rate differential that should, in theory, sit behind it.

Rough version of what I did: took the Bank of England rate minus the Bank of Japan rate, converted the annual differential into a daily amount per standard lot at the current price, and compared that with the broker's published long swap.

Results, roughly:

• The theoretical daily carry on a long lot came out noticeably higher than what the broker pays. My broker's long swap was a bit under two-thirds of the theoretical figure.
• The short swap was charged at more than the theoretical cost. So the broker keeps a margin on both sides, which isn't a surprise, but the size of it was.
• Two other brokers I checked had similar long swaps but very different short charges.

For a scalper this mostly doesn't matter, since I'm flat by the close. It matters the few times I hold a trade overnight, and for anyone building a carry trade argument around a positive swap.

Has anyone done the same check across several brokers? I'd be curious whether the markup is fairly standard or varies a lot by pair.