LONDON (Reuters) -North Sea investment firm Kistos on Tuesday walked away from its proposed offer to acquire British oil and gas group Serica Energy following weeks of wrangling. In July, Serica rejected Kistos’s cash and shares merger proposal, which valued the company at 1.04 billion pounds ($1.23 billion). Serica, which produces around 5% of Britain’s gas supplies, then made its own proposal to acquire Kistos, which Kistos rejected. Later in July, Serica rejected a revised merger proposal from Kistos at a valuation of nearly 1.2 billion pounds, saying it still undervalued the group. “Kistos…