Smaller companies are not obliged to publish their Equal Pay differentials.
No one is obliged to publish their equal pay differentials. What employers with over 250 employees have had to do since 2017 is report on their annual Gender Pay Gap - that's about averages, specifically the mean and median, and is not the same as equal pay.
Equal pay does feed into the gender pay gap - if you pay all your women 10% less, then their average pay will also be less, unless you had a weird situation where you only had women senior managers (and if that were the case, then it'd be unlikely women would routinely be paid less.) But you could pay all your staff equally (i.e. they get the same pay as others doing the same role,) and still have a big gender pay gap, because all your female employees are in low-paid manual and clerical roles, and all the senior management roles are held by men. If you've got just one token woman senior manager, if she's getting the same pay as a male peer, then they have equal pay, but if she's the only woman senior manager, it suggests they have other problems with equality.
Some countries have followed us in bringing in gender pay gap reporting. I think currently Iceland is the only country which reports on equal pay - since 2018, any employer with more than 25 employees has to have their equal pay status certified. I would like to think (and obviously Fawcett does, with the Equal Pay Bill) that other countries, including the UK, will follow suit, but we have a load of Brexit shit to sort out, and a Tory government, so I suspect it will take more than one attempt to make it into law. It's only had its first reading so far - see services.parliament.uk/bills/2019-20/equalpay.html to track its progress (you can do this for any other bill, too.)
(Guess who gave a talk on the history of equal pay last week...)