Looking at it from the offender's viewpoint (which I know isn't how you'd want to see it but it's how his barrister must look at it), his best chance of getting a relatively light sentence might be to try to get 'Section 18' (GBH with intent) dropped down to 'Section 20' (GBH). i.e. he could admit that he did strike out but state that he had no intention of causing such serious injuries. See this information, from the Crown Prosecution Service website, to read what adds 'with intent' onto a 'GBH' offence:
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/#a16
However if the medical evidence shows that such injuries could have only come about through a
repeated attack then his chances of getting 'with intent' dropped are probably nil.
When passing sentence in an assault case, a judge must consider the twin elements of 'harm' and 'culpability'. If he considers that both fall into the 'higher' classification he regard to offence as 'Category 1'. If one factor is seen as 'higher', with the other as 'lower', it's 'Category 2'. If both are at the 'lower' end of the scale, it's 'Category 3'.
If it can be established that a repeated attack was carried out, in a domestic situation, against a victim who took no active part in the violence, the judge might well consider that 'culpability' should be classed as 'higher'. The level of injuries which constitute 'GBH' are already significantly high, so they have to be exceptionally bad for 'harm' to also be pushed into the 'higher' classification. However injuries which might lead to permanent physical scarring (plus, quite possibly, psychological scarring too) could well qualify as 'higher'.
So (as I see it, based upon what you've written), the best the offender could hope for would be to sentenced at 'Category 2' level, with quite a high possibility that he'll be sentenced for a 'Category 1' offence.
The 'starting point' for a 'Category 2' offence (from which a judge must work up or down) is one of 6 years' imprisonment. Other than in very exceptional circumstances, a judge can't pass a sentence of lower than 5 years or greater than 9 years. Given that his previous convictions would probably push sentencing upwards, the offender's own guess of 7 years might prove to be remarkably accurate (if the offence is classed as 'Category 2').
If the offence is classed as 'Category 1' though, the starting point is one of 12 years' imprisonment, with an available range (other than in exceptional circumstances) of 9 to 16 years.
Note 1: Those sentences can be cut by one third for an early guilty plea (which there doesn't seem to be any sign of yet, based upon what you've written), or rather less for a guilty plea entered at a later stage.
Note 2: The actual time an offender spends 'banged up' is usually half of the nominal sentence period.
See pages 3 to 6 here to read what the judge has to take into account when passing sentence:
https://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf