Upton Marina at Upton On Severn now charge £15 to trailer launch on their slip, it is a flat charge that takes no account of boat size. A hand launch is £10, it is daylight robbery, on top of that the woman who sits in the office is unhelpful to say the least. The last time (and it will be the last time) that I launched there she complained that I was the second person who had interrupted her cheese sandwich eating and she was serious.
If you want to launch in the Tewkesbury area I can recommend the pub at Lower Lode, very nice people, £7.50, lots of parking and a nice pint, you can camp there in the summer and tie up to their jetty. There is a free town slip on the Avon actually in the town of Tewkesbury just down stream from the flour mill, parking can be a problem though, it is very easy to join the Severn through the manned lock a few hundred yards up stream from the slip.
If you are going to use the river between Gloucester and Tewkesbury beware of fallen trees etc.. floating in the river. A couple of months ago I couldn't believe the amount of debris in the river.