Good luck in your fractional gold stacking. I'm not buying or selling in the UK, so these are just some relevant, tangential thoughts.
When I jumped into fractional gold, I began with the 1/10 gold Eagles. But then I realized that they have a very high premium, around 15% usually. Acceptable if that's all I can find, but then I found the Mexican 5 peso (approximately 1/8 ounce), which typical can be had for about a 5% to 8% premium. So now that's my default impulse fractional buy. The 2 pesos (approximately 1/20 ounce) tend to have a higher premium.
In the sovereign category, I'll look at the pre-1933 half Eagles ($5 denomination), especially the pre-1908 Liberty head design. So far I've been able to find them for less than 6% premium. The quarter Eagles (half sovereign) tend to have a higher premium.
Main lesson is that the mid-sized fractions seem to be the best value.