I carried a guitar 4, maybe 5 times on flights since the new restrictions went into effect. In the US, the only new restriction is on anything liquid or gel being in your carryon. So- no toothpaste, no gel-based deoderant, no shampoo, etc.

I carried guitar right through from Chicago to Dublin to Hamburg last week. Granted, Dublin isn't UK; but it's pretty bloody close... :) So, guitar is generally ok on the US side; UK might give you some headaches...