I've been looking for ages! I read this book in the 1980s.
It was unfortunately one of those tacky, teen romance novels with a misty looking photo on the cover. The girl on the cover had pink hair...it was about her...she wanted to be a fashion designer and lived in London. She gets a job in a small clothing factory run by a grumpy, old fashioned man.
Her parents want her to go back to school after summer but she wants to carry on in the job instead. The title might have been "Something Summer"
I think it had summer in it anyway.
I have googled and googled. All I can find is that Heartlines was a series of books like this by Pan which were published in the UK. All the majority of the other lines were USA and this was defo very English.