Let's see if I've got this right? You plan on edge jointing five 130mm wide Steamed Beech boards to produce a 650mm wide kitchen worktop?
The first thing that occurs is that Beech isn't the most stable wood in the world. To get a permanently flat surface I'd prefer staved construction, ie where the thickness of each stave is approximately equal to the width. If you can stipulate quarter or rift sawn Beech, or if you can self select from waney edged boards, then that'll improve matters, but it's a banker's bet that commercial Beech will be flat sawn.
Second thing is that personally I try and avoid any biscuits or splines when edge jointing. If I've jointed two sub assemblies to the maximum width of my planer/thicknesser and I'm jointing those then I might sometimes pop in the odd Domino or biscuit for location, but by and large I've never found it necessary.
Regarding glue, I'd use good quality waterproof PVA like Titebond. The only concern isn't so much water as heat. PVA will let go once you get anywhere close to boiling. The big trade off you get with glues is joint lines versus other adhesive properties that you may want. PVA gives the best glue lines, certainly with the kind of clamping pressures you'll likely have at your disposal. Beech is a fairly pale timber, and nothing would look worse than a dark glue line.
I see you live on the Isle of Wight, if you ever take the Yarmouth ferry to the "north island" then at the other end you'll pass by my workshop. Feel free to drop in and I can show you examples of glue lines in a range of timbers with different adhesives.
Good luck!
