Check out Regent's Park in Central London - they have a lovely rose garden: http://www.royalparks.org.uk/parks/the-regents-park/

The obvious one is Queen Mary's Rose Garden, as jamikins says, and you may well find rosebeds here and there in different London parks.

Sissinghurst might be possible, but I don't know if they go in for late roses (best to check nearer the time): by train and bus, an hour and a bit from London:

http://www.nationaltrust.org.uk/things-to-see-and-do/page-2/item273775/
http://www.nationaltrust.org.uk/sissinghurst-castle/how-to-get-here/

I've just remembered, there is a rose garden area in Hyde Park, at the Hyde Park Corner entrance.

The Gardens of the Rose just outside St Albans are amazing and really interesting. You could get there by getting a train from London and then a taxi. However in Sept you'd need to be a member to visit...if nothing else, this website might be interesting to you.

http://www.rnrs.org/

Kew Gardens.

Regents Park for sure.

RHS Wisley. Instructions for getting there are on the website http://www.rhs.org.uk/gardens/wisley Plan on spending at least 4 or 5 hours at the gardens (so basically all day including travel time)

Queen Mary's in Regents Park, as others have mentioned, has beautiful roses.

thanks much for all the ideas. will check out the london spots listed above--
looking into sissinghurst and will wait to see whether the roses are still blooming when it's closer to the date.

i'm disappointed about needing to be a member for rhs-- looked at the web site and looks like it would be a fantastic excursion.

"i'm disappointed about needing to be a member for rhs-- "

You don't need to be a member. Wisley is totally open to the public. It is just that RHS members get in free.

oh-- i meant 'gardens of the rose'...members only in september.

If you want to attend - you can join the RHS.

I do that some years when I want to attend members' day at Chelsea.

janisj- thought about joining-- and would like to support them-- but it would be US $75 -- just too dear for this budget trip! maybe next time!

AFAIK It is only £38.25 ($60) - joining at this time of year they usually give a discount.

Still a bit of money but at least not $75.

"AFAIK It is only £38.25 ($60) - joining at this time of year they usually give a discount."

The tiny and badly cash-strapped Royal National Rose Society (which now allows access to its St Albans garden only for its members) costs non-UK residents £50 a year to join, and doesn't offer midseason discounts.

It's a completely different beast from the spectacularly well-pocketed Royal Horticultural Society, which operates a huge range of easily accessible displays around Britain.

janisj
i am showing 50gbp plus 5 joining fee at this address...???
https://www.netcomsecure.com/rnrs/

still-- probably too much either way with the train and taxi all figured in.