Hi Pete, given the limited speed (50 MPH) and limited duration (50 miles) for the use of a space saver wheel, the rolling circumference is less of an issue. Strictly speaking, the space saver is intended to get you to a place of safety where the correct road tyre can be replaced. Those VW Polos that you see driving around for weeks, and overtaking you on the dual carriageway, with the space saver fitted are doing it wrong.
Having said that, my own Range Rover Sport is fitted with 22-inch rims and comes with a factory installed space saver wheel (a relative term of course) that is fitted with a T195 / 70 / R20 116M tyre. Accordingly, there are 20-inch space savers available. Talk to the dealer.
If you do procure a space saver, then you will also need the remainder of the kit - jack, chocks, wheel brace, etc.
N6 JMX