Is there a reason why your parents might think the grandchildren are in more financial need than their own children?
For example, if some of the grandchildren are neurodiverse (autism, ADHD, dyslexia, epilepsy, etc) or have struggled financially or with mental health problems?
^^ My theory may not apply to all 6 grandchildren, but if it applies to 2 or more of the grandchildren, then perhaps your parents view their decision as helping to make life easier for certain grand kids who they know will struggle?
Do any of the 6 grandchildren already own their own homes?
Whilst it is a slightly unconventional choice your parents have made, you would be unable to challenge it legally. A family member only has a legitimate legal claim if they (a) were the spouse/partner, (b) were a child (inc adult children) living with the parent in the family home for some years before the parent dies, and (c) were to some extent financially dependent on the parent/s ..... and if meeting criteria (b) and (c) can then demonstrate in law that the amount left is inadequate.
You mention that your parents' estate is likely worth £1.1 million.
It might be more fair if your parents divided the estate 8 x ways (equal amounts to you, your brother, and all 6 grandchildren).
If we round down the estate value to £1million for easy maths that would work out as:
£1,000,000 ÷ 8 = £125,000 each
That would be £125,000 for all 6 grandchildren and the 2 x direct children.
^^ The only problem is, to effect this change requires you and your brother being in agreement and having a difficult and awkward conversation with your parents.
The other problem / potential issue your parents may not have considered in deciding that you and your brother only "need" £30K each is that:
● your circumstances (or your brothers) might change
● either of you may become ill or disabled later in life / unable to work / have mental breakdowns / divorce etc
Your parents have decided that you and your brother only "need" £30K now based on your current circumstances and current financial values.
If your parents survive another 20+ years the value of £30K may have severely diminished by 2055, and may only be worth £5K in today's spending terms.
^ Those are all things you could mention to your parents if you are brave enough to have the direct and difficult conversation with them about what is fair and equitable.