Andy,
get in a few local club races at the start of season (as near & cheap as possible) & you can pick up your signatures within 3/4 weeks, as well as getting in some extra practice. Remember, if it's a club race, it will only be a few laps to get your signatures (usually i.e. Mallory 7 laps) & if you pick the weekend meets, that could be 4 signatures in 9 days? & if you pick Mallory/ Donnington/Darley you know you have somewhere local to stop over for a break if you like ( a bit of luxury at no cost )........see ya!
ps. you don't need to race in a mono class (any class will do to get your signatures)