Hi! What are the differences if i use FSMone RSP vs IBKR to buy ETFs?

First of all, you have to know what ETF you plan to invest cause like you mentioned, FSM does not offer LSE.

Technically for IBKR, you have to RSP manually yourself cause they don't do GIRO deduction or any sort. However, what concern most of the people is the monthly fee of $10 commission minimum for accounts with $100,000 or less.

Assuming you only RSP $200 monthly into IBKR and being charged $10 monthly, the % fee is like ~5% which is not so cost effective. Whereas for FSM, the commission charge is $1 for ETF RSP. Of course, more variables to consider would be the FX spread and choices of ETF. FSM RSP ETF has limited choices as compared to IBKR.

Will only advise people to use IBKR from the start if they know they won't take long to achieve the $100,000 mark as they will not wasting money easily on the fees. Because other than that, I think IBKR win hands down in all areas.

When you said move on, you mean stop RSP and pick individual stock / asset class yourself? It depends on your investment commitment too as well. If you are too busy to allocate time for investment research, perhaps continuous RSP into ETF will be a better option.