Depends on your company.

In my case - I can submit a receipt with an out of allowance flight (e.g. a club flight) if I also submit a screenshot of the cost of the economy flight I'm actually claiming for (assuming it's less).

If you just don't want to raise questions in finance - sure - pay the additional £60 + cost supplement - BUT always be aware that the economy fare might not actually be changeable (make sure before booking) and that it's possible that in the time it takes you to book, wait for the receipt for work, then call to upgrade and get it processed the fare situation might change and the business fare will be significantly higher.

Personally - I wouldn't value a short haul business fare at anything higher than £30 more - but then I have lounge access already. Having said that though - lounges in outstations are generally not fantastic anyway.

Also be aware that if you're using a corporate travel agent - their version of the booking might change too. Whenever I UUA-ed my corporate travel agency bookings - it would update to show the new classes and reflect JUST the price paid for the UUA (which then got rectified for the invoices to finance).