meh, from my pov ep1 wasn't particularly realistic either - mainly in terms of timescales which again was due to the 1 shot thing
Katie was murdered at about half 10 at night and by the time of the arrest at 6am the next day they had multiple different lots of CCTV footage and had got an ID of Ryan
How? From where? Having been the person who goes round trying to get CCTV it is NOT a quick process - even in a murder case you don't go round knocking people's doors at 2am to get them up to look if their ring doorbell caught anything, you wait until the morning. Nearly all shops will be closed by 10pm and not open again until at least 6, and the final CCTV was "from the Council" - the whole point of CCTV is that it's REMOTE, there isn't anyone there watching it overnight - again any relevant staff will have logged off at 5pm and won't be wandering back in again until 8 or 9am. Bear in mind although the evidence was shown chronologically they'd had to have had the stabbing footage first to then know where/when/who they were looking for on all the other CCTV.
Even once they got the CCTV, who did they get to identify Jamie? Would Katie's parents have known a random kid in her year she wasn't friends with from grainy cctv footage? Even if they did manage to identify him (and not just as "um I think his name might be Jamie something?" but surname as well), they wouldn't know his address. 13 y/olds aren't on the electoral register, you'd be waiting again until 9am, either for the council to check child benefit or more likely to ask the school.
Bear in mind all this is happening overnight - while police are working, it's lower staffing (particularly for detectives) than during day hours, plus all the other crimes needing resourcing.
Then there's a risk assessment, getting a full squad of armed police in (6am would still be the night shift before earlies clock on), and giving them a briefing, making an interview plan, etc.
The DI acts as though he's doing Jamie a favour by recommending he ask for a solicitor when in the car but it's an essential part of his legal rights that the Custody Sergeant had to tell him anyway - if they'd interviewed him without telling him he was entitled to one a judge could rule the whole interview inadmissible evidence or even raised concerns about corruption/intimidation.
The duty solicitor seems to get there about 2 minutes after being called, and then randomly the DI tells him off for being too eager (for asking to see his client, again, something he not only is entitled to do but would be utterly weird if he didn't?!) and calls him a prick? Perhaps some officers do think solicitors are pricks but they don't insult them to their faces (particularly when they work with the same ones all the time!)
They should have formally questioned the dad to ensure he wasn't a potential witness or even suspect/accomplice first before allowing him to be the appropriate adult - the most likely place for Jamie to have got the knife would have been at home, same with the mum and sister, not allowing them to all travel to the station together and confer.
I'm also a bit iffy on the strip search - it's only required if there's reasonable grounds that the detainee may be concealing something. It's pretty obvious that Jamie won't have the knife hidden in his pants! They said something about seeing whether he might have had any other injuries, including on his penis (presumably defensive wounds from Katie?) - but they had the CCTV which showed the speed of the assault and that there wasn't any rape so how would she have ever been in a position to injure him through his clothing? Plus they shouldn't make detainees take all their clothes off at the same time but do top first, then put that back on, then bottoms off, to try and maintain more dignity.
Oh and they should all have had their BWV on before they even entered the house (or a separate officer specifically filming), and the DI should have kept his on throughout the car journey, for the purposes of confirming no unnecessary damage/brutality, etc.