There are two chemist shops (one in each direction along the street that the embassy is on - forget what it's called). Each charges either £3 or £5 to store items. (I think it's per hour, but might just be a one-off fee. Last time I went I paid for 10 minutes of storage time.
)
Also, be VERY liberal with what you store there. I went to the embassy without my phone, knowing their policy. I'd forgotten my Kindle was in my bag (fair enough), but even without that, security wanted me to store my security dongle for my bank (the type where you press a button and a security code appears that expires after a minute), and my earbud headphones (
no electricity involved in them at all, especially without any devices to plug them into!).
You may want to stop at the security desk first, have them flag whatever they won't allow, and then head over to store them. Both chemist shops are about 100 feet away from the embassy.