Santander has downgraded UK oil majors Royal Dutch Shell and BP, both primarily on valuation but also with the view that both are likely to struggle in 2015.The bank downgraded BP to an 'underweight' recommendation from 'hold', and cut Shell to 'hold' from its previous 'buy' stance.On the upside, the bank's updated model assumes a higher oil price this year, $56.50 per Brent crude barrel versus $52.50 before, and for both companies it has boosted its near- and medium-term downstream refining margin assumptions from its previous pessimistic view before.Furthermore, for both BP and Shell earnings forecasts were upgraded for this and the next two years.Moving onto Shell, Kenney had previously thought that it could weather a period of weak oil prices better than peers due to its balance sheet flexibility, but now thinks new strategic thinking discussed at the end of January lacks detail.There is "more urgency" needed by management to tie down its cash management efforts given still weak oil prices and even with divestment commitments and downstream offsets."We were surprised by the recent reintroduction of scrip offering for dividends from 1Q15 onward and think this could look like an afterthought that might signal more cash pressure than previously thought," he said.But the shares are worth holding for Shell's attractive dividend yield, with this likely to remain a priority for the company.