If you withdraw part way through the year, you may need to repay pro-rata part of your most recent loan disbursement. I'd generally suggest formally withdrawing either just before the next loan disbursement or during the summer to avoid any complication with this, based on experience. That said I gather they're usually amenable to payment plans if necessary if you do need to repay part of a disbursement pro-rata.
You will have to notify SFE of your withdrawal and the date you withdrew by calling them. When you reapply for finance for the new uni you'll just do it in the same manner but selecting the new course at the new uni. You won't "restart" your current application, you have to reapply for student finance every year of your degree anyway, and so you'll just be doing the same except selecting a different course as noted.
Bear in mind that you will now have used your "gift year" of SFE funding on the year at your current uni, so if you need to repeat a year at UCL or if you change course again, you may need to self fund that year. However you should still have enough funding to cover the full new degree.
Note that accepting an offer on UCAS has no bearing on your SFE funding - UCAS is a separate and unrelated process to student finance. Do take note of your offer conditions and whether they require you to complete the year at your current uni and/or receive a certain result for the year. You have to complete the year at this uni.