Can I enter the UK with another passport from the one I have a visa on? [closed]

So this can get a bit tricky.

I have a leave to enter visa currently on my US passport, which is now being renewed and can take up to 8 weeks to process. The passport is not with me as it had to be sent to the Home Office.

However, I am a dual Japanese citizen. I need to go back to Japan just for four days in 4 weeks, but when I come back to the UK, should I tell border control? A Japanese citizen can stay for 90 days on a tourist visa, I believe, and I have plans to go to Europe within four weeks of coming back to the UK, which means I can prove I will not overstay on the tourist visa.

So my questions are:

• Would this be legal given that I will not be entering with a passport that I have a valid visa on?
• And should I tell this to the UK Border Force that I have another passport with a visa that is in the process of being renewed, since I know they will ask questions.

Thank you!