I want to buy BTC at the cheapest rate available in London with cash
Always check the price and read the terms and conditions carefully. The rounding and price fluctuation might change the final amount. The fiat amount you enter matters and crypto amount is calculated at the time of request. If trading in person propose a meeting place which is safe to transact at.
Once you enter into a trade you will be able to chat freely with this trader through a private message box.
Once you enter into a trade you cannot automatically cancel the trade, as this may cause the other party to lose money. If a mistake has been made simply lodge a support ticket or start a dispute.