Arriving a few days before the start date

I already have my UK Ancestry visa with an official start date of July 1st; however I'm hoping to arrive in the UK a couple of days before.

Does anyone know if there are any problems or pitfalls with this plan?

Actually you won't be able to do that. The authorities will allow you in with the visa on the actual start date, otherwise they can deny you entry. This is why they advise people not to purchase a plane ticket before that date.