Brendon McCullum said England's strongest days are still to come as he begins preparations for a defining home Test summer, insisting his side is focused on evolving rather than dwelling on the Ashes defeat.

Back with the squad at a training camp ahead of next week's first Test against New Zealand at Lord's, McCullum outlined his belief that England can sharpen their aggressive "Bazball" philosophy and emerge stronger over the coming months. "You will see a lot of the stuff you've seen already, but with a bit of refinement," the England head coach said in an interview to ECB. "Positive and smart cricket can live together. We will try to step towards that.

"We will be brave when we need to be and smart when we need to be. We'll be totally committed and try to identify moments where games can be won or lost. If we can do that successfully we'll give ourselves a great chance. I'd like to see us become a team not only feared for our knockout punch but our jab too. If we can box smart we can achieve success and we're all fiercely driven to do that.

"There's some finessing and up-skilling to do, and maybe a firmer grip on things rather than expecting guys to work it out for themselves. We've had some good and bad times but I firmly believe the best years are still ahead of us and I'm delighted to still be at the helm for that."

England face New Zealand and Pakistan in a crucial home campaign, with McCullum adamant that both he and captain Ben Stokes remain united in their long-term vision despite scrutiny following the winter tour of Australia. Under the Stokes-McCullum leadership group, England haven't won a Test series against either Australia or India, whether at home or away.

"Stokesey and I have always got on well," McCullum said. "We challenge each other which is what you want and there will always be differences of opinion. That's healthy. The best leaders bounce off each other, but we're crystal clear on the direction we want this team to take and you'll continue to see us work together.

"We're absolutely fine and we've got each other's backs. There have been no 'clear the air' talks because there's no air to clear. We were all disappointed about the winter but all the talks have been about how to move forward and put that right."

McCullum also defended his decision to spend an extended period back home in New Zealand before returning to England, arguing the break had allowed him to reset and tend to his health ahead of a demanding season while still keeping close tabs on county cricket. "I've been able to have a little break after what was quite a demanding time," he said.

"I'm very lucky that, when Keysey (England managing director Rob Key) offered me the full-time role with the white-ball teams as well as the Test one, he was very mindful of the fact there would be a lot of demands with the schedule and that I had a young family. There would be times when I needed to be at home.

"I've been on the road for 25 years so I'm not unaccustomed to the demands of touring, but it is important at times to take your breaks, refresh a bit and get ready for an exciting summer ahead."

Addressing further criticism of his belated arrival, McCullum also revealed that part of his absence was due to medical treatment. "I'm not after pity but there were a couple of operations I had to have back home which have been booked in for a long time, so it's nice to tidy that up," he explained.