@Jean, I think that Mark Seymour knows what he is doing.
The seasons are getting even more wonky with the changing climate. I have been a bit non plussed the last two years with what has been said on the forum about the apparent lateness of the gardens in London and the South East, because they have been reported on here as being many weeks later than we are achieving 300 miles north in the Lake District.
In my innocence I would have expected it to be the other way around.
Now I am a very amateur gardener at a tiny little Community garden but we are a multi Green Flag award winning garden (the only such garden in the whole of Cumbria)- a distinction we share with Seward Park in Seattle (on my cancelled trip this year one of my stops was to be at Seward Park). While Seward Park is on an utterly different scale, our Green Flag judge this year was from Kew Gardens, so we got a good grilling and inspection from someone with knowledge and skills we will never have- but we also got very valuable learning tips from him.
It is just a bit nerve wracking to get a judge from Kew!! And we aren't in it for awards (nice as they are) but the difference to the community. Having said that I can't deny that awards day is a key date in my Calendar.
We are planting to achieve colour all the way through from very early Spring (February with luck) to late Autumn (early November with luck), a part of that is with varietal roses throughout the season. This week we have been judged by Britain in Bloom (as part of an entry for the whole town)- they will have seen a very different garden than when Green Flag came.
Obviously you will see different things with Mark (even in the same garden) than you will with garden tours a few weeks later (the same as you will do in our humble little garden), but I honestly can't see that it will be any less good.
Ultimately it depends what you are expecting to see in a garden, as regards to when you come.
What you will miss seeing on a Guided Tour is the National Garden Scheme gardens- private gardens open for a few days a year- which can be the match to any public garden, and are raising money for Charity. They are all across the UK.