No, you don't. You can make a no comment interview.
I suppose in principle you could refuse to attend but then they will just come and arrest you.
If you don't consent to the BAC calculation they will just do it covertly using the specimen of breathe you have provided.
Generally speaking, you want to be answering questions if you have an innocent account.
If not then you should be making a no comment interview.
Can I clarify anything for you?
Not if it was taken on behalf of the police. Blood is always taken by medical professionals. The police don't do that in custody.
They don't need a warrant for you arrest. They have powers under PACE that are stand alone.
Probably because they are investigating an offence arising from the accident.
Also, they probably don't accept that you can't remember.