The 28-year-old scored 19 goals in 43 appearances for Isthmian League North Division side Maldon & Tiptree last season.
Barnwell joined the Jammers in the summer of 2020.
He made a goalscoring return to Isthmian Premier side Bishop`s Stortford for a second spell on August Bank Holiday Monday 2019 in a 3-0 win at Kingstonian.
He originally played for Clapton and Romford before his career took him to Bowers & Pitsea.
He then re-joined Dagenham & Redbridge in the 2016/17 campaign during which time he was loaned to Solihull Moors and whilst there had a short loan spell at Lewes.
He left the Midlands club to link up with Barking in September of that year.
After short spells at Brentwood Town, Bowers & Pitsea, Ashford United and Grays Athletic, he joined Great Wakering Rovers in the latter part of the 2018/19 season and helped the Southend side avoid relegation from the Isthmian North.