If they're found guilty in Romania, would they have to serve their sentence there and then be extradited to the UK to have another trial, and possibly serve another sentence?
This is probably Romania looking for some diplomatic back scratching. It really depends on what agreement the UK and Romania come to. Also there are a number of other outcomes that are deep in the negotiations.
According to the blurb, they'll be transferred to the UK after the trial in Romania. Even if they're found guilty in Romania first. They'll then have a trial in the UK. If they're also found guilty there, Romania and the UK will come up with an agreement on which prison gets him first.
Maybe UK wants first crack? I don't think there's anything stopping Romania from extraditing them, then once they've (hopefully) served their sentences, requesting they be extradited back.