Are you sure you are telling us everything?
Take a look at the Magistrates’ sentencing guidelines:
https://www.sentencingcouncil.org.uk/publications/item/magistrates-court-sentencing-guidelines/
Click on “Magistrates’ Court Sentencing Guidelines” and then go to page 307 of the document. There you will see categories of “Culpability” and I would, from your description, place yours in “C” (Opportunist One-off offence; very little or no planning). Then go to the following page where the “Harm” is categorised by the amount involved. Yours is in Cat. 4 (£5k to £20k). Then the following page provides a table of Culpability and Harm and recommended sentence ranges. For Cat C Culpability and Cat 4 Harm the sentencing range is a Band B Fine (which is one week’s net income) to a High Level Community order. The offence (at least as you describe it) does not reach the custody threshold and certainly does not warrant the matter being heard at the Crown Court. Did you enter your guilty plea at the Magistrates’ Court and the Magistrates decided it was outside their sentencing powers? If so, there must be more to the offence than you are telling us about.
Anyway, you are where you are and if the guidelines are followed you should be sentenced to a Community order of some sort. You will not have to pay all the prosecution costs. A guilty plea in the Magistrates’ Court attracts a contribution towards costs of just £85 and I believe it would be a similar amount if the case goes to the Crown Court for sentencing.