Great to see that there is interest here in preserved stationary steam! It has always been an interest of mine and I saw both Kempton Park and Mill Meece in their working days in the seventies. Along with over a dozen colliery winding engines!
Beyond those already mentioned, the list of further sites well worth a visit - mostly pumping stations - is actually quite lengthy - just off the top of my head:-
London and the South:-
Tower Bridge
Crossness
Crofton (with Great Western connection!)
Eastney, Portsmouth
Twyford
East Pool, Cornwall
Levant, Cornwall
East Midlands:-
Abbey, Leicester
Clay Mills, Burton on Trent (8 large beam engines between these two sites!)
Papplewick, North Notts
Cheddars Lane, Cambridge
and some more modest examples:-
Lea Wood, Cromford
Pinchbeck, Spalding
Stretham Old Engine, Cambridge
and a couple smaller examples from the pottery industry:-
Jesse's Bone Mill Etruria,
Gladstone Pottery, Longport
and a colliery winder:-
Bestwood N Notts.
West Midlands:-
Bratch, Wolverhampton
Sandfields, Lichfield
Coleham, Shrewsbury
North East:-
Ryhope
In the North West, there are a few impressive examples remaining from the textile industry:-
Trencherfield Mill, Wigan
Ellenroad Ring Mill, Newhey
Queenstreet Mill Burnley
Wiseman Street Mill, Burnley
Nutters Shed, Barnoldswick
And also a huge colliery winding engine at Astley Green near Leigh, and one at Haig Colliery near Whitehaven
A few museums worth a visit for staionary steam
MOSI, Manchester, (Power Hall being refurbished until next year)
Wollaton Park Nottingham
Markham Grange Nuseries, Doncaster
(Science Museum London also stole a mill engine from Burnley!)
Appologies to the many which have slipped my mind!
Not every one of the above runs under steam but a web search will fill in the details. They all need all the support they can get - I have often thought it ironic that in railway preservation, we have been building "replicas" of steam locomotives whilst, at exactly the same time, we were scrapping a number of large and impressive stationary engines.
Hope that is of interest,
Howard