She will need to attend CFO seminar first coz its one of the requirements for her status to be changed. But when I applied to change status on my passport, the officer didn't ask for my CFO certificate. I just submitted our NSO marriage certificate(original) and my previous passport, I opted the 10 days priority service for Php1,200. I received my new passport with my husband's surname on it after 7 days so I was pleased with it.
I don't think she will need to take another test, she can present marriage certificate as proof of her identity.
If she still have sometime before she submit visa application, I suggest she should change her status on passport now, she will need to change it when she's in UK anyway so better to do it now. The DFA priority service takes 10 days or less.