My wife and I are in the process of instructing a tax advisor. She arrived on a spousal visa last June and I am a UK national.
A colleague used to use US Global Tax (which is on your the list Durhamlad pointed you to) and was happy with them. They do both US and UK tax returns. I contacted them and was quoted £800 plus VAT for a federal return, £350 plus VAT for a state tax return and £550 plus VAT for the UK return.
As my wife's salary is low (and was reduced considerably when her employer switched her from being paid by the US office to the UK office) this was too expensive.
So we looked at Greenback who seemed reasonable. You are correct that they only do US taxes but they work with a UK tax advisor and you can find out details of her prices here:
https://www.greenbacktaxservices.com/partners/uk-self-assessment-tax-return/?utm_source=internal&utm_medium=email&utm_campaign=greenback_csc,
In the end,however, we have decided to go with another recommendation, Olivier Wagner at 1040 Abroad.
https://1040abroad.com/blog/author/olivier/.
He is based in Canada but seems to know his stuff. His fees were 400 CAD for the federal return, 100 CAD for the State, and 100 CAD for an FBAR return. We had a 20 minute video consultation with him and he is clearly knowledgable. His TrustPilot scores seem pretty good too. I do not think he gets involved with UK tax returns but hopefully I will be able to manage that.