I used Equidiner to order some Horsehage when my local feed store ran out over winter. Fast delivery, and it only cost me £3 more to have it delivered than buying locally! I think they've recently changed their delivery prices, but would still be worth a look I think.
I have used Farm and Pet Place occasionnally. The courrier sends you a text to tell you at which time your order will be delivered. Delivery is 2.99 for a 20kg bag (you can add little things like wormer, cat food without added postage cost but it goes up quickly if you order several bags of horse feed). I believe their linseed is the best price around as well.
If I had to put in a larger order, I would consider Simple Systems, they do a free delivery for orders over 80 quids. I haven't used them yet though.
I use efeed and they've been really good, very helpful and feed normally arrives within 3 days! suits me as finding his chaff is like gold dust (dengie healthy tummy). Since they changed to parcel force they've been wonderful, as I order 2 bags at a time.