Whilst you are in bristol, try and make time to visit 'M shed';
I haven't yet been myself but as it is very close to the Great Britain it may be worth a look.
On your way from London you could visit Didcot (http://www.didcotrailwaycentre.org.uk/) and/or 'Steam' at Swindon (http://www.steam-museum.org.uk/).
Of course, there are many other preserved steam railways etc in the vicinity - probably more to choose from than you will have time for! Have a look at the HRA website (http://www.heritagerailways.com/)
I know you say you are going to Kew but if you are really into pumping engines then Kempton Park is a must (http://www.kemptonsteam.org/).
finally - for something different (if you are interested in 'all things mechanical') you could try the musical museum whilst you are Kew(http://www.musicalmuseum.co.uk/)
I could go on but hopefully these may give some suggestions.
What days are you at Dorset? I will be there on Wednesday evening and all day Thursday.