The first test for the CPS, when determining the appropriate charge, is to examine the level of injuries. You injuries are consistent with those for which a charge of 'GBH' would be deemed appropriate.
Once that has been decided upon, the next thing that the CPS has to consider is to whether adding 'with intent' to the charge (taking it from 'Section 20' to 'Section 18) is appropriate. As I read it, the 'intent' element was present, therefore the more serious 'Section 18' charge will be used:
https://www.cps.gov.uk/legal-guidance/offences-against-person-incorporating-charging-standard
You ask if the charge is likely to stick. Based upon the information you've given to us, the answer would seem undoubtedly to be 'Yes'.
When determining the appropriate sentence, the judge has to consider both 'harm' and 'culpability, to determine whether each of those factors should be classed as higher or lower. That will provide him/her with a 'starting point' sentence, from which he/she can vary the sentence upwards or downwards but only within fixed parameters.
As your former partner kicked you, that's likely to push 'culpability' into the 'higher' classification, because a shod foot is regarded as a 'weapon' for sentencing purposes.
The 'harm' factor
might be seen as 'lower' though because, despite the level of your injuries, they still weren't as serious as the very worst GBH cases are (where, for example, the victim suffers brain injuries that leave them requiring 24 hour care for the rest of their lives).
With one of the two factors being 'higher' but the other being 'lower', the judge has to work up or down from a 'starting point' sentence of 6 years imprisonment for an offender who was convicted after a trial or 4 years for an offender who pleaded guilty at the earliest opportunity.
For an offender found guilty after a trial (with one 'higher' and one 'lower' factor being taken into account), the minimum sentence a judge can pass is one of 5 years imprisonment, with the maximum being 9 years.
For an offender who has pleaded guilty at the first opportunity (again, with one 'higher' and one 'lower' factor), the minimum sentence is one of 3 years 4 months, whereas the maximum is 6 years.
Sentences of above 2 years imprisonment can't be suspended. They must always result in immediate custody. The actual time spent in prison is usually half of the nominal sentence, with the offender being 'on licence' for the rest of the term.
Based upon the limited information we've got (because, for example, we don't know about any previous offences), and assuming your ex partner pleads guilty straight away, my best guess is that your ex partner will be sentenced to 4 years imprisonment, meaning that he'll actually be 'banged up' for 2 years.
https://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf