I have recently traded 5%, 10% or more of daily turnover on some low liquidity stocks all over the world and I don't seem to have observed any significant price impact. Many of the low liquidity stocks have good market makers so it seems easy to get a lot of liquidity at low cost around the midpoint price. I even tried trading 2/3 of the average daily turnover on a Mexican stock with less than 1% price impact because I just took the liquidity by market order and the spread was less than 2%. For me, the commissions seem like a bigger problem than the market impact.
It seems that, even through IB, I can easily access liquidity equivalent to 5-10% of average daily turnover on AIM shares without incurring a price impact cost of more than 1%. The situation seems not so serious as you described before.
Re: UK - You must be buying stuff that isn't on the SETSqx exchange. Usually the SETSqx stuff is less liquid and you can only buy/sell them during short auction intervals throughout the day, unless you use a more UK centric broker with better MM access.
Unfortunately I don't know of a way to filter out SETSqx stocks from P123 models so it's tough to isolate them when you're trying to run backtests and whatnot.
Of course, if you're indeed able to work with the limited IBKR trading times of the setsqx stocks, and find that the price impact isn't great, then that's excellent.
I now realise that for companies that only have half-yearly reports, the Q stands for half a year rather than a quarter. But somehow my unadjusted accordingly, selected factor list based on the backtesting results of the US microcap universe (i.e., dan's factor list) can still work better in the European stock market than in the US stock market.