Some
halfords tyres have max pressure 40psi, I'd suggest higher. Also halfords tyres not terribly puncture proof. I would suggest Schwalbe marathon, or marathon plus. Not that much more expensive.
That's proper money for a tyre, so I'd be looking at a respected proper make one such as Schwalbe Durano, or Marathon or Continental gator skins etc. Of course, the Halfords one may be a proper brand tyre with their name on it, but you are guessing whether it's good or not (my guess, would be "not", but it is only a guess).
You may already know this, but you don't need any tread at all for the road, wet or dry - thus "slicks" rather than "semi slicks".
Panniers and rack are indeed good thing if you need to carry anything much, albeit quite expensive for decent stuff, as are mudguards if they can be fitted.
I'd read up on the Great Helmet Debate" before shelling out any money on one. Whilst I'm not going to re-tread that here, I will just say "obvious" is not necessarily equal to "true".
Special shorts, jackets etc - well, OK, a "good thing" and I've got 'em, but old clothes are much cheaper !
Clip in pedals and (unfortunately) special shoes are a very good thing, but give it a go with what you've got first as this is an investment- albeit can be kept if you do upgrade bike at some point - and not everyone gets on with clip ins. Toe clips are a cheaper alternative, and avoid needing special shoes, although trickier in use.
One "extravagance" I do recommend is a track pump - although fatish mountain bike style tyres don't need to be pumped up as hard as road tyres, it is a lot lot easier to do so with a track pump.
At the risk of teaching my grandmother to suck eggs, do get the saddle height right - likely higher than you might initially think. There are rules of thumb for this on the web.
