The opposing solicitor will never reply to you if you email them, they'll just take you off their reply saying "I do not take instructions from your client" and you'll just waste people's time.
As said above, this is the biggest purchase you will make and going cheap is a false economy.
A few hundred quid extra now could be a good investment in the long run. Online firms are just bucket shops with one or two solicitors overseeing dozens of paralegals relying on churn for profits...