Lay at BSP for a fixed stake rather than liability

Hi

I may be missing something obvious, but I would like to be able to lay multiple selections at BSP for a fixed unit stake, say £2. 

The Lay at SP action only seems to allow this for a fixed liability.

Any ideas gratefully received.

Thanks
Pete

Hi
I think this is because of BetFair's SP minimum
copied from BetFair SP rules:
The minimum liability for a bet placed at BetFair SP is £10 for a lay bet and £2 for a back bet (or the currency equivalent in each case)
so you can only place a pre-off SP bet using a liability

or, you could lay at actual_sp as soon as the market turns in-play, but no guarantee of getting matched.