Building Regs do not preclude the use of heat recovery systems and this will allow you to dispense with vents altogether. In the case of Lisam, the vents can be replaced with an openable vent (with hit and miss cover). The TGD Part J states that: "background ventilation (may be provided by a) wall or window ventilator with a controllable ventilation grille". As a simple fix the existing vent covers can be simply replaced with the contollable vents. I wouldn't recommend that anyone blocks up their vents with towels or the like- the vents are there for a purpose. If you block vents up permanently condensation may form within the dwelling and cause damage to the structure or contents, unless alternative arrangements are made to ensure that proper ventilation is available.