I'm up near Berwick most weekends (I've a caravan in that area) - if you come unstuck I'd have no problem picking stuff up for you from the local fishing shops around here and bringing it up with me.
That said, if you're going off the rocks (in a manner of speaking) you could simply pick up some of Sainsbury's finest shelled mussel (1.99 a pack) and they do fresh Mackerel at the wet fish counter too.
Berwick hasn't got a Sainsbury's but the Morrisons branch up there has a wet fish counter where you can usually get fresh mackerel. They had plenty in there last weekend and this morning.
Mackerel is one of my favourite baits - it's relatively inexpensive, pretty tough and you can cut the fillets you take from the fish into any size you want, making it very versatile since you can go smaller and thinner for small flattie baits through to an entire fillet (or most of one) if you fancy your chances for conger, tope or rays, depending on the locality you intend fishing obviously.
I use mackerel almost exclusively when rock fishing, normally on a two-hook pennel rig, or in smaller, longer pieces on lighter gear when float fishing for pollack.
There are places up there where you can dig your own worms or pick a bunch of mussels and shell them if that blows your skirt up.
If you're a lazy hack (like me) you can cheat by using a combination of supermaket fresh fish and tackle shop baits.
Don't overlook frozen baits though - bluey is a fantastic frozen bait, as are sandeels. I clip the heads off sandeels and use bait elastic to wrap two of them together into a single bait, usually on a 3/0 or 4/0 pennel, depending on the size of the eels. Sometimes you need to use thinner shank hooks like Aberdeens when using sandeel, but it's a great rough ground bait.
If I had my choice of any bait for the rough stuff I'd probably go with mackerel and fresh mussel.
FYI I've seen conger caught at Dunstanburgh and Craster, and another NESA member tells me he has seen Ling caught at Dunstanburgh as well, so you could be in with a similar chance at St Abbs.
Finally, the jetty/quay at St Abbs harbour is great if you can get on there before anyone else shows up. The crane the've put at the end makes casting a chore but basically there's a kelp bed between the end of the jetty and the central channel where the boats come in and out. You can drop heavy gear into the kelp or a forty(ish) yard cast gets you into the middle channel. I've seen some clonker pollack pulled in there.
Best of luck with your trip
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