I recently bought a little over 3ks worth, so been in your position. I did it sporadically and definitely bought badly initially, meaning I paid too much for some silver coins. But, not too bothered as in the long run, should be fine.
Many mention HGM metals for sovereigns, and while it is cheaper on the face of it, when you add postage in, you can actually pick them up cheaper on Atkinsons (free delivery), for both half and full? Am I missing something with that? (just checked and it does vary. Sometimes its Atkinsons, sometimes HGM). Full sovereigns right now are I think about £2 cheaper on HGM.
It does seem to go up and down, but Atkinsons has always come out cheapest every time I have bought anything.
Keep an eye on the "Pre-Owned" section for silver coins as you can pick them up VAT free from time to time and at very good prices over spot (depending on volume). If you buy new you are paying a hefty chunk on VAT and will be wayyy over spot. I also bought from CelticGold.eu as well as you can pick up VAT FREE when you do that for silver.
Sovereigns are good on all fronts, as CGT free, and as many have said above (that know well more than me), hold value well. I have one half sovereign but do want more now as my silver has grown.
Also, if buying purely for weight, keep a spreadsheet with your average cost per ounce. As you buy more ounces, especially if buying regularly, it is very handy to know what your average cost per ounce is, so you can look for opportunities to buy lower than that, thus bringing the average of the whole stack down.
Good luck only spending that initial 3k! You will want more.