Hi
I commute by train from Brighton to London Bridge and walk the remaining 4km to my office in North London - takes me and my long legs about 35 mins. I'm planning on getting a folder for this journey and tried out a Dahon Speed D7 (2006 model) and found it to be quite "flexy", particularly at the handlebars when going uphill (not standing on the pedals). I'm 6'6" and quite heavy so perhaps the entry-level Dahon wasn't for me. I'm now looking at trying a higher-end Mu P8 which supposedly has a stiffer frame but it isn't yet available in the UK until mid-March I'm told.
Make sure you try before you buy and check if your employer will allow you to buy the bike using the Ride2Work government scheme to offset the cost of the bike against tax - you'll save at least 30% on the RRP of the bike.
Cheers
Johno