It may impact your application, but it depends on what the switch is from/to. If you're switching from, e.g. maths to engineering, the latter will usually be satisfied that failing at uni maths will translate to doing at least reasonable well at uni engineering (as the nature of the work isn't as difficult as it is extensive). If you're switching from say, marine biology to plant biology...it may be held against you.
Also, you may want to speak with the administrative and/or student support staff at your university; there may be courses you're interested in at your current university you can transfer into internally without a UCAS application (although you'll need to reapply for SFE under the new course details). Additionally if either of those options fail, you can always have a look in clearing later on.