First off, need to correct your income figures…
If UK goes after Borneo, they will indeed be up 4 there. But they WILL lose India in J1, and that will make them only net +1.
Also, if Japan chose, they COULD re-take Borneo on J1. Their SZ60 TRN can reach, as can their entire fleet and the majority of their air force. The added advantage of killing off the UK fleet in one place, including the FIG in all likelyhood, still being able to attack China and do Pearl medium is also a positive trade for Japan.
Not saying that re-taking Borneo on J1 is a good idea… I personally would wait until about J3 to do so… once I have my mainland IC, have taken care of teh US naval threat, and established a good shuttle of forces into Asia. But with a Borneo move early by UK, I get India for nearly free, no counter of Germany taking Egypt in G1, and a nice chunk of my Japan fleet is poised to then move on and take Australia and New Zealand earlier than I normally would do so.
It is a nice quick land grab for UK in an area they normally don;t get to go after. But the trade offs for the Allies make it a detrimental move on UK1. If you REALLY want Borneo, do it a few rounds later using Aussie forces, as well as any naval air units that survive the re-take of Egypt and any Japan naval countes in the Indian Ocean.