Your 5 year count to ILR does not start until the day you enter the UK, so there is no benefit to entering earlier or later, except for your own personal preference and circumstances.
The earlier you enter, the earlier you can qualify for ILR, but that's about it.
i.e. if you enter the UK on July 23rd 2019, you will qualify for ILR on July 23rd 2024. If you enter on, say, August 20th 2019, you will qualify for ILR on August 20th 2024.
Personally, I would go for the cheaper flights and travel a few days later. As long as you enter the UK before the 30 days are up, that's all that matters.