Re: USA to UK
One of my in-laws families have settled in the US and I'd say we're better off in England.
Be prepared for the weather-it is kind of unpredictable but I'd miss the rain if it didn't appear for a while. We do get heatwaves which when they do hit they are really hot (according to my mum these heatwaves create a 'dry summer feeling what that means in beyond me).
If your friends in-laws are community focused and attend loads of local Eid parties, dawats etc she'll make plenty of friends and find her feet quickly.
As for clothes-erm it's not a UK requirement that all people must wear jeans. Girls wear what they feel comfortable in. I personally have a very mixed wardrobe which reflects by shopping addiction.