JPAsp

It is recommended that the i-130 is not filed when you are in the USA.   USCIS sometimes gets confused and thinks you are trying to adjust your status vs going thru the NVC and embassy.   And if they get confused,  that will cause a delay in you process.

Ultimately, you can travel if you show enough proof that you won't overstay/adjust status on whichever visa you use (Inc VWP). At the moment we can fly as spouses of US citizens over the travel ban. Some people have had issues with using their ESTAs for some strange reason and have had to immediately apply for another one to resume travel. I think these may just be flukes. I have travelled twice to see my husband since our application and have had nil concerns. This was with an ESTA and the most recent travel was for the month of June. I had enough proof of return if they had asked, like job and return ticket etc. Therefore planning to stay for an extended time with no real ties to the UK may work against you. Repeat shorter trips may be better, or itineraries with an end date.
 
Like with all travel to the states, there are always risks in being permitted entry however not necessarily due to an ongoing spousal visa.

Madrid, Spain is in fact open for interviews. I'm not sure on other countries. Call your embassy if you can, if not, email them. Madrid is very quick to reply, usually same day. Also on the embassy's website it should state if they have resumed IV interviews or not. 

Yes! We file March 5 2020 and it was approved and received NVC Case Number etc by June 10, 2020! I have no idea why it's so fast, but I'm not complaining at all!