Soon for VAT and Income Tax you will need to use "compatible software" from an approved vendor, you won't be allowed to just submit your details through the governments own website anymore. Great stuff.
https://www.gov.uk/guidance/find-software-thats-compatible-w...
https://developer.service.hmrc.gov.uk/api-documentation/docs...
And because of that there's tonnes of approved software, including loads of actually-free services:
https://www.tax.service.gov.uk/making-tax-digital-software
Including GNUCash, where you can register for your own developer key and file away for free - https://github.com/cybermaggedon/gnucash-uk-vat
This seems like exactly the right way for a government to opt out of writing its own web front-ends.
More seriously - and after my knee-jerk has died down, my government actually publishing APIs does seem "pretty-schweet"