Does graduating a UK PhD in 5 years look bad (with circumstance)
I'm not in the UK, but if you would accept that your question is "Does graduating with a PhD 1 year later than normal look bad?" I would say no, it doesn't, and especially if it results in 1 additional good publication, it might even look better. As @ProfSantaClaus mentions, publication records are typically most important.

Additionally, having received a presumably competitive grant looks good, doing work with an additional research group looks good, and expanding your network by working with another research group is also good.

I wouldn't worry too much about this as long as the whole process makes good sense for you in other ways, and it sounds like it does.