I always try to be in England in May or October. Not only is the countryside looking spectacular, but I've always found the weather to be pretty decent.

Of course, we are talking about Engalnd, where you have to expect the rain and go out prepared.  

Anni: I really like traveling in the UK in October and even into November as the tourist crowds have really thinned out. A couple of things to be aware of during that time, however, are:

Autumn half-term usually occurs about the 3rd week of October when UK families book accommodations like mad (and prices go way up for that week!). Best remedy: book early and if you are going to stay in one area for a time, you can sometimes have the higher price for the half-term week lowered if you book a cottage for two consecutive weeks--cottage owners are willing to compromise on price if they have a firm booking well ahead of time ("bird in the hand" and all that!). Same goes for B&B--a very early booking during half-term periods might get you a better rate rather than waiting. Also keep in mind other half-term periods during the year which will affect accommodation pricing.

Many of the great houses close at the end of October, some even in September (though their gardens are often open year 'round), so you might not want to travel after November 1st. If you have a smart phone, both the National Trust and English Heritage have apps that you can download onto your phone with information on all of their attractions. There are also phone apps for CADW (Heritage in Wales) and VisitScotland Days Out (encompassing Historic Scotland and the National Trust for Scotland).

I've traveled in the UK in every month except for September and each season has its advantages. I'm sure that you will have a splendid time there no matter when you go!