It is essentially what you've listed. This is lighter than the Type 26, which--among other things--has a Strike Length VLS which can fire things like Tomahawk. From the outset, the RN has indicated it wanted a lower-end, lower cost alternative to the (very expensive) Type 26. The Type 31 program has always been aimed at this requirement from its inception. It is cost capped at £250 million. The RN was to have built 13 Type 26 in 2 variants (a higher end, and lower end version.) Instead, it cut the Type 26 numbers to 8, and plans (at least) 5 of these new low-end ships. They cost less, and their armament reflects this. However, as you've correctly pointed out, there is room for expansion/upgrades at a future time if/when budgets are better, or need requires.
So, the specialist has "correctly" noted that the Type 31 is armed lightly (below potential) but has no particular clue why, apparently. Every navy would love to afford/equip their fleets with the finest high end ships available always. Budgets get in the way of this. It has long been understood in the profession that--if you have hulls--then when a war breaks out, funds for weapons upgrades start to flow. But, you have to have those hulls. If the government offers funds for an under armed hull with room to grow, you say "Thank you!" and get building!