If you're an employee (as opposed to freelancer/self-employed), the date on which you get paid (along with other terms) is stipulated in your employment contract and an employer cannot change your pay day without notice and consultation period.
And yes, it is a major sign that they are "in financial difficulties". If the employer does go into liquidation, there is a strict order in which creditors get paid. The first to be paid is ... you've guessed it ... the liquidator. Next are employees up to £800 each, then the Government and creditors who hold charges over assets, then the ordinary trade creditors.
Don't allow your exposure to salary to be more than £800 'cos the chances are you won't get it.
However, the business could go into administration where a professional insolvency practitioner goes in to try and rescue the company. This usually works by a new phoenix company being set up and continuing to trade whilst the debts of the old company are frozen until the new company has made enough in order to pay off the debts.
It would be up to you whether you have the faith to continue in order to help the business recover or whether to jump ship now.
Personally though, I think an explanation would be in order. If your payday has purely be altered for convenience, then your employer has a legal requirement to give you notice. If it's because they haven't got the money to pay you ....
Some managers have the philosophy that you tell your staff nothing as ignorance is bliss and they need the staff to continue the business. Others would be honest and ask for loyalty and support. I'd guess yours fall into the former category but both are risky strategies.
But it may well be worth putting feelers out for another job.
Cheers
Whirls