Sherlock had given Mycroft's men the slip in Italy. He knew they'd been trailing him and quite frankly, it was annoying. He didn't need to be looked after by his big brother. He was perfectly capable of taking care of himself. He'd been given a tip off that one of Mycroft's best henchmen had made a return to England and the more he heard about it from various sources, the more he realised he had to get back to England too, and as quickly as possible.
Very few people knew that Sherlock was still alive. In theory, it was only supposed to be Molly and Mycroft. He'd been working undercover on the Continent, using various different aliases to help out the local police and bring justice to members of Moriarty's criminal web. He was quite alarmed when, upon catching one of Moriarty's henchmen in Northern Italy, the man muttered "I know who you are Sherlock Holmes...." Sadly the man had been dying at the time and could offer no further information despite Sherlock's best efforts at last minute interrogation, but since then Sherlock had been on high alert. He had dispatched spies and allies to keep their ear to the ground and gradually he became aware that word was slowly spreading through the remaining members of Moriarty's gang that Sherlock Holmes was indeed alive and well. He knew it would only be a matter of time before they made a move to try and force him out of hiding.
Sherlock heard about John's accident from Mycroft. His first instinct was to return immediately but in the end opted to wait a little longer, to see if John regained any of his memory. He wasn't sure whether it would be bad or good for John to remember Sherlock was dead. After careful consideration he decided it would be good. He wanted to see the look on John's face when he found out Sherlock wasn't dead. It would be priceless, he thought with a small grin.
But when he heard that one of Mycroft's henchmen had been sighted in England, things began to fall into place in Sherlock's mind. They knew Sherlock was alive. They'd heard about John's accident. They knew John was ill and vulnerable. They knew if they could get to John, Sherlock would re-emerge from the shadows to protect his friend. They were quite right. And Sherlock figured if they could find out that much, they could easily find out where John was staying. He blamed a security leak. It was obvious Moriarty had someone working in the British Government, with access to Mycroft, access to classified information regarding Sherlock and his colleagues. Moriarty practically had men everywhere. This was only confirmed when a member of his Homeless Network informed him of a new sighting in Cheltenham, the location of his brother's mansion. Sherlock booked himself on the next flight out.