I disagree that you should 'let them sort themselves out at 15'. Your daughter is still a child- legally, socially, and definitely emotionally. She needs parental guidance and protection. Especially if there's a predatory-seeming man that's worming his way into her life.
I would do 2 things- definitely talk to DD. Just because you want the situation to go away doesn't mean it just will. She's not stupid, she sees what's going on, and it's affecting her directly, and with or without your involvement or approval it will keep escalating (the bf and dad messing with her and putting pressure on her). Use this as an opportunity to teach her how to make good, healthy, long-term choices. Trust her enough to be able to discuss this with her and have her see why you have to take step 2.
Step 2- cut off all communication with these people. Explain to your daughter why she can't be in touch with her bf (because of his dad) and how/why it could be dangerous for her. Tell her that if the dad ever messages her again after she's blocked him on a different platform, etc. to come straight to you, and tell your DH to definitely confront the boyfriend's father and let him know in no uncertain terms to leave your family and child alone. Don't shy away from explaining to DD what the consequences of this continuing could be- don't be afraid to use the correct language like 'potential grooming', etc. This step is also why it's so important to discuss this with her first- if you just cut them out without her input, she'll feel like you're trying to unfairly control her, and like you're treating her like a kid (which she is, but teenagers really dont think that they are) and this could then lead to a full-fledged rebellion that could actually endanger her academic progress, etc.
You have to get her away from them but in a way that she sees and understands why it's necessary and agrees with you.
And look, I know the whole 'we want no distractions, she has to focus on GCSEs' thing seems like the plan, but this is life-sometimes things don't happen on a tidy timeline for your convenience. This is a potentially dangerous situation, it's definitely already inappropriate and dodgy, so GCSEs or not, you're gonna have to deal with it with your DD. Plus, this is resilience buliding- very rarely in life do we get to just focus on our main goal without other responsibilities, issues, etc. also requiring our attention. This is a perfect way how you can help guide her on what to do when things happen at inconvenient times. If she'll have your support, she'll be able to handle it (but if you don't talk to her about, and she tries to muddle through this on her own with the creepy dad of BF then it could really go off the rails). Plus, this is a great chance for you to help her understand what healthy relationships are, what boundaries are, what's inappropriate and what red flags mean/look like- for a young woman, that's a potentially life-saving lesson. You have a chance to really help your DD not have to deal with this situation on her own- use it!
Ultimately, tldr- you may want this to go away, not encourage it, etc. but there are other people involved (bf and his dad) and they aren't just gonna drop it without your (serious) intervention. Gonna have to deal with it one way or another, so involve your daughter, help her feel like she has choice and power and is allowed to prioritise her feelings, and do it right.