Hi Inejenn and hi to everyone,
I have looked at the census returns on Ancestry uk, and yes Peter is present at 1841 and 1851 with his son Peter also present at 1841.
Also it is listed he died 1855.
I think you are right not looking into age at time of 1841 census because it is true they rounded age off as opposed to exact age so the 1851 census is more reliable. The fact he was listed as a grocer at 1841 is most likely what he actually did for work and that it has him as shopkeeper (pauper) at 1851 confirms his occupation. It is more than likely listing what he did and what is status was at the time of survey: pauper i.e. no income. He was 91 years old and probably incaple of work.
I would get his death certificate it may not give you his town of origin but it may give you his address at the time of death, and if this is a workhouse then there will be more records for you to seach at the record office. It may be worth trying to obtain the church/parish record he married Hannah in because info may also lie their.
Where did you get he was a merchant from? because if he was then it may be worth trawling French websites because most Italians came to England through the ancient trade routes (Roman times) through France as opposed to sailing especially merchants. It is also not uncommon for merchant from Italy to import fruit into England so there maybe a link to his occupation in England. Below is a link to genealogy website which brings quite a few Paulucci's up with origins in France.It may be worth contacting the originators of the postings
http://www.geneanet.org/
Also I have included a link to a project ran by Leicester University, which is getting historical directories online. There are quite alot already. It will help to verify occupation and where he lived over a number of years. The website is not always the most stable but is well worth persevering with
http://www.historicaldirectories.org/hd/index.asp
Hope this is of some help