I've come across these so far;
DT Brown http://www.dtbrownseeds.co.uk/
"Take advantage of our special FREE Postage & Packing Long Weekend by entering the promotional code D16MAY when you check out online – it's valid up until midnight on Monday 2nd May 2016. Can't be used with any other offers."
Dobies http://www.dobies.co.uk/ free standard deliver until midnight bank holiday Monday, enter code DOBPP16
Mr Fothergills http://www.mr-fothergills.co.uk/Amazing-Garden-Seed-Sale/?ccode=F16ESEED#.VyO4_PkrKUk £1 seeds offer, just enter F16SEED at checkout.
I'm not 100% if it all works together, but in the email they sent me, they seem to also be offering
-buy any 5 packets and get the cheapest FREE!
-PLUS 2 FREE packets of seed with every seed order†. Free seeds are Cosmos Purity and Tomato Garden Pearl.