If you want a set to just have forever, I'd pick the Arthur Price classic Rattail set, because if you lose the teaspoons or want to add steak knives in 6 years time you will be able to and they will be identical.
They won't be making the Sophie Conran Rivelin set forever.
I have had an Arthur Price set for a long time.
Mine is Harley not Rattail, but both are classic patterns you will always be able to get.
Over time I've ended up with all sorts of additions to my original basic set, including picking up a few extra places settings, and I've had to replace the odd fork that somehow disappeared during a house move.
I also picked up an extra set of teaspoons as we go through a lot.
On the Arthur Price website under sample service, you can pick a set of 5 different styles of teaspoon from their range for £5 + postage (you get one of each style you have picked), so you can see what you think and compare them.
I tend to wait until November to buy extra bits from their site, as they have a big sale on everything for the whole month every year.