Most people training for a half marathon will do their longest run (whether it's the full HM distance or not) about two weeks to 10 days before race day - so around 10 September for Ealing Half Marathon on 24 September. It’s generally accepted that it takes 10 days to 4 weeks to benefit from a run so running further in the final 10 days will only make you more tired before race day without providing any benefit. Undertrained but fresh on race day is better.
If you can, it's worth making this longest run a rehearsal for race day on the basis of (Tip 7) "Nothing new on race day".
Try out the full kit you will wear on race day to make sure nothing chafes or rubs in the wrong places
If you're intending to use gels, test your strategy
If you can start your run at 9am, that will allow you to think about what you might have for breakfast or for dinner the night before.
Any other things to include in your race day rehearsal (and perhaps to purchase or to source in the weeks / days leading up to your dress rehearsal?