You absolutely can decline antenatal care but you need to understand what it is you are declining and be sure you don't want it or if you want it part of it. You could ask to be referred to the consultant midwife to discuss this and make an individualised care plan for yourself and your baby.
Routine midwife appointments involve screening your urine for sugar and protein and usually signs of infection. Sugar can indicate gestational diabetes which is more likely in older mothers and later pregnancies, would you want screening and treatment for this or not?
Protein can indicate pre eclampsia which is often symptomless, it's less likely in a subsequent pregnancy with the same partner but higher if a new partner and also older maternal age.
Asymptomatic UTI can lead to preterm labour or late miscarriage.
You could buy urine sticks and screen your own urine.
Routine blood pressure checks screen for pre eclampsia as well. Will you take your own blood pressure? Or do you prefer not to screen? Are you aware of symptoms although it can be asymptomatic
Later midwife checks also monitor fetal growth, it's not the most accurate way and you could opt for four weekly private scans instead but if that's not your choice, do you feel comfortable with not monitoring fetal growth. Later in pregnancy they also look at position, a scan would replace that.
The other routine checks are blood tests, one is group which means they can cross match rapidly if you have a significant bleed post birth and also screens for antibodies which are rare. The other looks at haemoglobin and platelets primarily. Are you comfortable not knowing these? Or would you like the blood tests.
The 20 week scan screens for anomalies, my daughter's terminal condition was picked up at this scan, would you rather not know if the baby had significant anomalies even if that impacted on their survival or will you opt for the scan or a private alternative?
Declining care is and should be perfectly fine but you should know what risks you are comfortable with and what care you do want so you can seek it out. If you're planning home birth, many hospitals have a home birth care who provide continuity of midwife care at home and you may prefer that. I'd really encourage you to engage with the hospital and discuss what you want and what they can offer so you can get a care package in place that is just right for you.