As Andy indicates, the police most definitely should run a check on whether or not your brother was insured to drive your car at the time of the speeding offence. However, with the system under great pressure, it's just possible that they might not.
If they do, your brother will be prosecuted for driving without insurance and you'll also be prosecuted for allowing him to do so. (The fact that you had an insurance policy in place is irrelevant). You'll both get 6 points on your licence and you'll both be fined 150% of your 'relevant weekly income' (which is basically your take home pay unless you're, say, unemployed, in which case it's £120).
[The number of points can actually be up to 8 but I see no reason as why the 'standard' 6 wouldn't apply here. Similarly, there could also be some variation in the amount of the fine but, once again, I see nothing which would make the magistrates vary from the 'norm'].