selenia 20k is not an ordinary semi synth. its called a Tri synthetic oil, basically means it has better shear ratings then ordinary semi synth oil.
Selenia Racing is a 10w60 fully synth oil, and is actually an excellent lubricant, but i echo the comments that it is simply not necessary. If you doing trackdays often id advise a fully synth, but for normal road or even fast road use the semi synth is Perfect.
And £75 is a good price, the selenia 20k semi synth is approx £28 for 5L + a genuine filter would be £8.
Selenia racing oil will cost you a whopping £48 for 5L, so £75 is not bad at all.
Use a branded oil either way, Shell, Castrol, selenia, Mobil or Motul are the Best names in the business. Halfords own brands are similar to Comma oils and other lower market lubes, they are not true semi synth, that are called a part synthetic, which is NOT the same.