@Gerard The plugin doesn’t support that kind of compound condition.
You could create two campaigns, one sent to the regex segment, the second sent to UK and not L4 and not sent the first campaign, but then you would have two sets of statistics to look at.
It might be possible to send the campaign to the UK and not L4 segment first, then when it has finished sending edit the segment condition to the regex and resubmit. phplist won’t send the campaign to a subscriber who has already received it.