The last weekend in August is a national holiday period, so Ambleside will be very busy. However, this need not spoil a trip when you are aware of it and plan accordingly. Public transport is available in Ambleside but it doesn't directly access the best parts of the Lake District, so renting a car would be better and allow you the freedom to avoid the crowds. In general the Lake District south and east of Ambleside, more accessible from the M6 motorway, will be very busy in the holiday period. Travelling north from Ambleside (and this is possible by bus number 505 to Keswick) will take you away from the crowds. Even better, by going north-west by car you will reach the quieter walking areas on the fells (i.e. mountains). Parking may be difficult at popular sights, but the Lake District beauty is available to view at many quieter locations.
On the Tuesday after the holiday weekend Ambleside returns to being a much more pleasant area. This is most notably seen in the friendlier attitude of your local hosts, who will be much less stressed.
After the Lake District, a trip to Wales (particularly north Wales) would provide scenery very similar to the Lake District, with walking trails of similar standards. The weather will also be much the same, so the risk of rain is just as high.
The Cotswolds would provide a complete contrast, with gentler walking (although longer walks will be just as tiring). The villages, and their pubs appear more frequently and there is some chance that September weather could be warmer and drier.
If you enjoy hiking, eating in public houses, and meeting friendly locals then your vacation hopes will be met in all these places. Remember that the British can be at their best when the conditions are less than perfect.