YANBU but it might be better, as some people have said, to wait until your child is older before deciding.
Schools are a very artificial environment. Nowhere else in society are people compelled to spend time with others with whom they might have nothing in common besides being born in the same year. Children naturally learn from others (including older children) who are more experienced, and can consolidate that knowledge through passing it on. The comment made a couple of pages back about homeschooled children seeming 'older' and this being perceived as a negative thing is very interesting - if anything I think social development is hindered by not having a more organic, mixed-age group to socialise with.
That said, school is in many ways the default option in our society, and many children get on well with and enjoy school. If a child is an extrovert, and developmentally average, they're likely to get on very well at school and enjoy it. If they're likely to be significantly ahead of or behind their peers academically, school won't cater well for them, academically or socially. Children who are ahead will be held back, and those who are behind will be made to feel like failures instead of being allowed to develop at their own rate. Introverted or anxious children may find school overwhelming, especially with the current test-focused climate. School teaches conformity - if a child is the type to fit in with their peer group (and I don't mean that in any way pejoratively, it's just an observation) they may not even notice this, but if they're different in any way, it can be difficult for them.
Not being in school at 5, or taking formal tests at 7, will not impact your child's ability to sit GCSE or other exams as a teenager. It's as silly as the notion that young children need to be dressed identically at school to prepare them for dressing appropriately in the workplace in their twenties! All the homeschooled children that I've known have been able to sit exams either as external candidates or by choosing to attend college as a teenager, and they've done very well.
The view expressed by some that you shouldn't homeschool because 'your child might seem different to others' is an incredibly narrow-minded one. As if difference were a fault needing to be stamped out by schools! But 18 months is a little early to make a decision and ideally you should choose based on your child's temperament and learning needs - whether that's homeschooling, a particular state school with an ethos you support, or private if that's an option.
(For context - school did not suit me at all, for various reasons, and my personal experiences were extremely negative even as a small child. I've since worked in early years and KS1 and seen some children really thrive and flourish in the school environment, and others for whom school is a constant struggle - not because the teachers and other professionals are incompetent or uncaring in the slightest, but because of the environment and the structure of the system. I don't have children yet myself but the concept of childhood within education, particularly home education, is a research interest of mine.)