The problem with fixed limits, especially in money laundering cases, if there is a fixed limit, people will find out, and abuse it. Say you had a £30k limit in the uk, and I was wanting to launder through you, I would do £29k, then get a mate to do £29k, then another mate, then another and so on.
What I meant by a risk to you is, and I'll delete this if you think I should, but if I won £10k from you and you sent me a SoW. You keep accepting deposits, so I could deposit say another £3k, if I win, send you my SoW and get paid. If I lose, refuse to send it, then you have to reset the account to the point when you sent the SoW so I would get the £10k owing at that point, plus my £3k in deposits refunded, so a free go in effect. Like what was happening with SE cases a while back.