Yes you can view it online & if you are clever you can copy it to your computer. As well as registering you need to buy some credits, you will need (5 credits) £2.00 to view the certificate. What you get is an uncertified certificate which is 100% as good as a certified copy which is £15.00.
When you have your credits go to Homepage & under Search registrations select search for a death registration, the rest is self explanatory. When you find the right entry there are 3 options, Enhanced which is a partial transcription, Full which is the certificate for 5 credits, Order certificate is the £15.00 version. So select Full.
Unfortunately civil registration of marriages commenced in 1845 & births & deaths from 1864. With Thomas jnr being born in 1839 his birth & parents marriage were long before civil registration so church records are your only option to get back further.
Looking at the 1861 census entry for Thomas Jnr for clues, did you see he is down as the nephew of the head of house “John Wreen”. A very uncommon name and very difficult to read on Ancestry, I really wonder if it is not something else. If you use FindMyPast see what name they are reading it as. The importance of this is that it may indicate the maiden of his mother.
Do you have Thomas snr’s 1889 death cert? I have looked but can’t see it.
tc