Quite frustratingly, there isn't [publicly available] official guidance that clearly and explicitly states that you definitely can or cannot enter the UK as a visitor while a visa application is processing. That being said, here are some things to think about:
You cannot submit multiple UK visa applications simultaneously
When you present yourself at the UK border, have a brief chat with the IO, and get a stamp in your passport - you have technically applied for a visa. It's a very informal visa application, but that stamp in your passport is your visitor visa and your chat with the IO is your application. US citizens have the benefit of not being required to formally apply for advance entry clearance to visit, and can essentially be granted a visitor visa on the spot upon presentation to the IO.
If you have a settlement visa processing, the view may be taken that by applying for a visitor visa, you are effectively withdrawing your previously submitted settlement visa application or varying it to a visitor visa application instead. Or, if you have a settlement visa processing, you wouldn't be permitted to apply for another UK visa, and you would just be refused entry.
You cannot switch status in the UK as a visitor.
When in the UK as a visitor, you are not permitted to switch to a spouse visa. It becomes a bit of a tricky area if your settlement visa is decided and approved when you are physically in the UK, even if you submitted the application when you were physically in the US.
Some take the view that if you are in the UK on the day your application is approved, this will cause problems as it could be argued that you have, effectively, switched status while present in the UK as a visitor - which is expressly not permitted.
So, overall, these could be the potential outcomes of trying to enter the UK while your settlement application is in-progress:
- You are permitted entry with no negative repercussions for your in-progress application.
- You are permitted entry, but at the cost of voiding your in-progress application so you would have to do it all over again (including paying the fee again).
- You preserve your in-progress application at the cost of refused entry to the UK and associated problems with that (IOs questioning you about it even after you've been granted a visa, needing to declare it in applications for visas to other countries, etc).
IMO, unless you have very compelling reasons to visit the UK while you have an application processing, I just wouldn't risk it. If you want to spend time in the UK, I would suggest doing so before submitting the application.
