Reporting capital gains online

For the purpose of completing reporting capital gains tax for residential property, one of the questions while reporting capital gains tax asks the following question ' What was the market value of the property when you gave it away?' as the transfer was a transfer of equity whereby I added my child to my deed as joint tenants to a property which I was the sole owner of, do I answer the question by submitting 50% value of the property; i.e the market value of the proportion I gave away? for example, if the property value at the time of the transfer of equity was £200,000 would I have to report a value of £100,000 in order for calculations to be correct?

Hi
Yes that is correct.  
You will use the market value at the date the half share was transferred to your child.  
Please see Capital Gains Tax: what you pay it on, rates and allowances: Market value - GOV.UK (www.gov.uk).