UK ETF USD/GDP?

Hi everyone,
Since I have started to work quite recently, I am now opening an investing account.
I am looking for a very standard passive long term investing behavior with much focus on S&P500.
After some research, I found out that two of the best etf available for UK investors are the IShares and Vanguard ones.
I opened an account with Invest Engine (super pricing terms) and I wanted to invest in a accumulating etf.
I noticed that only CSP1 (Accumulating in GDP) is available, while CSPX (Accumulating in USD) is not available on that platform and on others such a s Free Trade.
Since I believe the pound will eventually strengthen in relation with the dollar, I would like to invest in the USD Etf (I want to avoid the currency hedged one).

Why am I not able to find CSPX in the available etfs?
Since it is listed in the LSE, shouldn’t it be available to UK investors?
(I know that in the long term of 15-20 plus years currency differences are not very relevant, but it is also a curiosity for me)

Thank you in advance!