As I see it, you're wrong to assume that there's automatically no CGT to pay upon the sale of your property because you've classed it as your main residence. The fact that you've let it out removes your right to a full exemption from CGT.
However letting relief will apply instead. That's likely to be £80,000 (if you and your husband co-owned the property or £40,000 if only you owned it) which can be knocked off the capital gain before tax is calculated. Further, there might be an element of private residence relief to be taken into account (to allow for the gain in the value of the property while you were actually living there).
My answer is based upon reading these links:
https://www.gov.uk/tax-sell-home
http://www.lawpack.co.uk/tax/property-tax/articles/article7648.asp
(See 'Let property')
https://www.informaccounting.co.uk/blog/what-is-lettings-relief