I could recommend many areas of the UK but at that time it is very likely the "best" places will be full of people. Many of the better locations are full of people at weekends so you will need to start early or late if you want to avoid lots of people in your pictures. During the week is better but like you I have another job to do. Not sure how far you want to travel from London - but the UK isn't too huge anyway so you can still reach most of the UK within reasonable traveling time.
My suggestions would be:
Derbyshire Peak District - not far from Sheffield which is 2 hours by train from London. My personal favourite but I do live very near! Can get very busy when weather is good at weekends.
Lake District - big landscapes available (but can get very busy at weekends during). About 4-5 hours travel from London, depending where you go in the area.
Sussex Downs. More pastoral than the above, but can nonetheless be dramatic. Easy to get to from London by train - 2 hours max.
Norfolk Coast and Norfolk Broads (that's rivers, not another meaning!) Much flatter but again a great area for smaller landscapes if that's your thing. 2 hours train from London to Norwich.
A couple of other places to look for some ideas:
The UK's National Parks are the wilder places we have in the UK. Start at
www.nationalparks.gov.ukThe BBC Picture of Britain web site
www.bbc.co.uk/arts/pictureofbritain divides the UK into several areas which can be browsed to get some ideas
If train travel is your thing you can get an indication of journey times using the National Rail enquiries web site
www.nationalrail.co.ukThe National Trust is the custodian of large amounts of land and historic buildings -
www.nationaltrust.org.uk There are many sites close to London.
It's difficult to make recommendations without really knowing what sort of thing might be of interest and there is so much that the UK has to offer! I hope this provides a start to think about but no doubt others will have a bunch of favourites.