Allowed, yes, in that there's no law against it. However it's quite difficult! Summon up all your stick-to-it and you can get it done.
Three things that helped me were:
(1) Working with a mortgage broker. You don't need a bespoke one, mine was just the one attached to the estate agent that was selling my flat. They can fight in your corner. (And if they don't, you should get another one.)
(2) I had about two years of employment history built up. Not self-employment. Although that can work too, if you have at least two years of steady income to show in your tax returns.
(3) I found a good solicitor. DON'T use the one attached to the estate agent. Definitely don't use the "online only" solicitors, as one of them below us in the chain nearly ruined the whole chain of purchases! Get a local recommendation from someone who's recently purchased a house, if possible. And don't go with the cheapest quotation either, as you get what you pay for!
Good luck... it's worth it!