Police and Criminal Evidence Act means anyone that gets arrested for an applicable offence (of which assault is one) has DNA* and fingerprints taken. It also confers the right to take those things by force if necessary due to refusal.
If you are subsequently not convicted, you can apply to have them removed from the database.
*we only take DNA once, if you are already DNA confirmed on the system it's not taken again. Fingerprints are taken every time.
+1
PACE was also endorsed and extended by CPIA. Also if a person has no prior conviction or caution against them and they are released with no further action by the Police, are not cautioned or convicted by a court for that arrest, then their DNA and Fingerprints must be destroyed under the Protection of Freedoms Act. An act of law which has sharp teeth against the Police if not complied with fully and promptly upon notification.
I'm not a cop, never have been, but I've seen footage and TV of people resisting arrest or seeking to be awkward for them and e.g. football stewards; I have been one of them, even in training situations, deliberately physically awkward people are very hard to deal with safely on your own. Injuries can easily happen to anyone if a person is lashing out etc & it can take a number of people to protect everyone from harm.
Whatever the right or wrong of the arrest may be (and whether they have decided to treat him to a Life on Mars reenactment or not) the cops are doing what they are required to do having chosen that route. He seems unaware of their legal rights towards arrestees and has chosen a confrontational route himself: biting through plastic swabs is his choice but he's then presenting them with a problem to manage as he's creating at least a choking hazard to himself that the cops have a duty of care to protect him from without getting their fingers bitten off in the process.
If they're wearing riot gear (hmmm) when they deal with deliberately physically awkward people then fair play, I'd rather them be protected today to be on duty tomorrow, rather than off sick with a broken nose 'cos they've caught a flailing arm to the face or are having a pinky stitched back on.
Or are suspended on full pay for ages while someone investigates how a prisoner died in custody.