It sounds to me that what you are seeking is to place your pensions into a SIPP (Self Invested Personal Pension), is that correct? Investment options are usually very wide.
If so, there are plenty of UK SIPP providers, however many restrict access to USC's - some could argue (myself included) that this is discriminatory but good luck trying to take that one on! There are a few however that don't seem to have slammed the door shut.
I'm a USC/UK and have a SIPP with IWeb - which is part of Halifax Share Dealing and is administered by AJ Bell. I chose this because of low fees on higher amounts - their fees are fixed compared to many providers who charge a %age of fund value. You can trade online, a little bit clunky, but it has worked for me - biggest drawback is no App, but then at least I'm not paying for it.
Previously I've had a SIPP with Hargeaves Lansdown (Good but expensive with hidden fees), Fidelity Investments (was good, but they abruptly slammed the door shut on me being a USC when I tried to execute a trade while in the US!). I tried to look at Vanguard when they launched their SIPP and being a US based company felt it would be a good solution - perversely however they don't accept USC's!.
Here's a useful article (not written with USC's in mind however) that gives a useful table of charges - might be a good place to start:
https://moneytothemasses.com/saving-for-your-future/pensions/the-best-cheapest-sipps-low-cost-diy-pensionsIt's really case of searching and then trawling through their T&C's to see if they restrict access.
Good luck, let us know how you get on.