To be frank, I usually find these ££ or €€€ indicators pretty useless. I could wander into your place and have a wrap and a beer for pennies - alternatively I could have a starter, a main and a bottle of wine and it'd be five times the cost of the wrap and beer. The same principle applies to any restaurant that has a wide ranging menu.
If they are enquiries from Facebook then personally I'd reply with a brief message "we have many dishes from £x through to £xx, with main courses starting from from £x". And in a quick note about the exchange rate and that should cover it, adds the personal touch as well and gives you something to do once you've finished a nice 16hr day. An alternative is to put a copy of the menu (with prices) online.
If I was pushed, I'd go for £ or ££ for MB.
JF