avast is designed to be a 'resident' AV so what you are suggesting isn't what it was designed for and as such would require lots of hacking to get it to act like an on-demand AV.
The avastUI has no security function, but is there so you can access the various shields and settings.
Eventually we got the information out of you as to why you want to do this - to resolve a perceived web browsing problem:
I do want the protection, but I'm suspicious that avast is the cause of some web browsing problems i've been having, and I want to close it to see if it is.
So why not let us try and find and resolve that problem. The web shield is the shield directly related to the internet, so as suggested you can stop the web shield, whilst still retaining the protection of the other shields.
What are the web browsing problems ?