I've looked through the T&C's and don't see any restrictions on overseas investors.
I'll preface this post by saying 2.58% is much, much better than 0.20%, so don't take my comments in the wrong way.
Since the US is attempting to replace Switzerland as the worlds largest tax haven, there may well be no restrictions on residence or nationality. But it would be worth attempting to register online to find out 2 things: first, do you require a US address, and second, do you require a US bank account.
As for the 2.58% return, some thoughts:
I'm not sure of the implications for HMRC reporting (when to declare profits or do they qualify for a UK tax free amount). If tax is due in the UK, and offsetting any US tax paid, for those in the 40% UK bracket especially the after tax profit won't match what one would receive in the US. Still, ~2.58% is better than 0.20%.
Many US expats in the UK do not have a US address or US bank account. They may prefer to have their assets in the UK since they live here, pay bills here, buy things here, give gifts here, or plan for leaving inheritances to those who live here. Investing in anything in the US carries a degree of uncertainty for them. The original funds will be in £s, and it will need to be sent to the US dependant on exchange rates. Growth is constant in the US, but always varies when compared to £s. If the funds are to end up back in the UK, then there is the exchange rate influence on what the funds are worth at that time plus transfer back to £s.
IF the intention is to have the funds and profits to forever more stay in the US (moving back, inheritance, gifts to the US, etc.), than this may be an interesting plan, but for those who intend to live forever more in the UK with no need for funds in the US, some considerations may be required. ~STILL, 2.58% is better than 0.20% and exchange rates can go down as well as up.
Just to add:
As pointed out above, there are UK accounts which pay much more than 0.20%, so the decision as to where the funds are versus a 0.75% gain in interest rate versus varying exchange rates may need thought. Is it worth it? (it may well be worth it for some)